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This'll probably be the end of my cleanup quest for the day, but it's
a pretty pleasing result.  More cleaned up more easily than I expected.

Decided a basic bool property for this is probably fine after all.
(For quite a while I'd been wondering if we'd end up with an enum
of variants that are more or less semantic siblings of Repr, but it
still hasn't happened, and thus it's now seeming quite unlikely.)

This one turns out to be different betweens scalars and recursives,
if in a small (and fortunately consistent) way.

Some are easier than others.

I'm starting to evolve a (still ad-hoc) naming convention for the
extracted functions in order to provide some cues for how common you can
expect the thing to be.  Scalars have a lot more commonizable stuff
than non-scalars, for example.

Also, hej: now recursive maps can be compiled!  (Previously they were
stymied simply because the maybe emission hadn't been pasted in.)

In research: I found that there are more low-cost ways to switch which
methods are available to call on a value than I thought.  Also, these
techniques work even for methods of the same name.  This is going to
improve some code in NodeAssemblers significantly -- there are several
situations where this will let us reuse existing pieces of memory
instead of needing to allocate new ones; even the basicnode package
now already needs updates to improve this.  It's also going to make
returning representation nodes from our typed nodes *significantly*
easier and and lower in performance costs.  (Before finding methodsets
are in fact so feasible, I was afraid this was going to require our
typed nodes to embed yet another small struct with a pointer back to
themselves so we can have amortized availability of value that contains
the representation's logic for the Node interface... which while it
certainly would've worked, would've definitely made me sigh deeply.)
Quite exciting for several reasons; only wish I'd noticed this earlier.

Also in research: I found a novel way to make it (I believe) impossible
to create zero values of a type, whilst also making a symbol available
for it in other packages, so that we can do type assertions, etc,
with that symbol.  This is neat.  We're gonna use this to make sure
that types in your schema package can never be created without passing
through any validation logic that the user applies.

In codegen: lots of files disappear.  I'm doing a tabula rasa workflow.
(A bunch of the old files stick around in my working directory, and
are being... "inspirational"... but everything is getting whitelisted
before any of it ports over to the new commits.  This is an effective
way to force myself to do things like naming consistency re-checks
across the board.  And there's *very* little that's getting zero change
since the changes to pointer strategy and assembler interface are so
sweeping, so... there's very little reason *not* to tabula rasa.)

Strings are reimplemented already.  *With* representations.

Most of the codegen interfaces stay roughly the same so far.
I've exported more things this time around.

Lots of "mixins" based on lessons learned in the prior joust.
(Also a bunch of those kind-based rejections look *much* nicer now,
since we also made those standard across the other node packages.)

Some parts of the symbol munging still up in the air a bit.
I think I'm going to go for getting all the infrastructure in place
for allowing symbol-rename adjunct configuration this time.
(I doubt I'll wire it all the way up to real usable configuration yet,
but it'll be nice to get as many of the interventions as possible into
topologically the right places to minimize future effort required.)

There's a HACKME_wip.md file which contains some other notes on
priorities/goals/lessoned-learned-now-being-applied in this rewrite
which may contain some information about what's changing at a higher
level than trying to track the diffs.  (But, caveat: I'm not really
writing it for an audience; more my own tracking.  So, it comes with
no guarantee it will make sense or be useful.)

fill in generated type for list

Fill in generated types for int, link, and bytes nodes

Fix traversal internals to use it (!) rather than converting segments
to strings, which was both wasteful, and in some cases, *wrong* (!)
(although by coincidence happened to mostly work at present because of
another thing from early-days code that was also technically wrong).

Fix ipldfree.Node to reject LookupString if used on a list node!
(This is the other "wrong" thing that made the traversal coincidentally
work.)

LookupSegment method generation also added to codegen.
Signed-off-by: Eric Myhre <hash@exultant.us>

So far everything we've used these mixins for was generating based on
a *schema.Type*.  This isn't true anymore: representation nodes are
still nodes, so most of the same helpers are useful for the same
reasons and we want to reuse them... but then now they're not always
directly related to a particular reified schema.Type anymore.

Correspondingly, these helper templates were using the .Type.Kind
property as a shortcut to avoid more args, but that is wrong now:
for example, if we're going to generate the representation node for
a struct with a stringjoin representation, the closest thing we would
have to a schema.Type is the struct; but that kind clearly isn't right.
So, now that is another property passed internally to the embeddable
helpers (the helper-helpers, I guess?), and affixed by the helper
rather than provided by a schema.Type param (which, again, we no
longer have).

Note for more future work: this is the first time that a "TypeIdent"
property has shown up explicitly, but it's also *all over* in the
templates in a defacto way.  So far, my policy has been that extracting
consts from the templates isn't a priority until it proves it has a
reason to also be handled *outside* the immediate locality of the
templates (because if we pulled *everything* out the templates will
become a thin unreadable soup; and doing bulk sed-like edits to the
templates is fairly easy as long as they're consistent).  Now, that
criteria is probably satisfied, so more refactors on this are probably
due very soon.

And one more further sub-note on that note: I'm not actually sure if
some of these types should have a "_" prefix on them or not.
"$T_NodeBuilder" currently doesn't -- meaning it's exported -- and that
might not be desirable.  I started them that way, so for now I'm just
sticking with it, but it's a thing that deserves consideration.
(It might be nice for godoc readability if there's a clear hint that
the builders exist; but also, at present, there's no guarantee that
their zero values are at all usable, and that should be cause for
concern.  Other considerations may also exist; I haven't attepmted to
weigh them all yet.)
Signed-off-by: Eric Myhre <hash@exultant.us>

In general, picking a consistent strategy with this error and sticking
with it.  The strings were starting to accumulate too much ~stuff~,
otherwise.

It might not be a bad idea to upgrade the MethodName strings to a bunch
of consts, but I don't think it will get any harder to do that in the
future if we should so choose either, so, meh.
Signed-off-by: Eric Myhre <hash@exultant.us>

Signed-off-by: Eric Myhre <hash@exultant.us>

Most important things first!  To follow this refactor:

```
sed s/TraverseField/LookupString/g
sed s/TraverseIndex/LookupIndex/g
```

It is *literally* a sed-refactor in complexity.

---

Now, details.

This has been pending for a while, and there is some discussion in
https://github.com/ipld/go-ipld-prime/issues/22 .

In short, "Traversal" seemed like a mouthful;
"Field" was always a misnomer here;
and we've discovered several other methods that we *should* have
in the area as well, which necessitated a thought about placement.

In this commit, only the renames are applied, but you can also see
the outlines of two new methods in the Node interface, as comments.
These will be added in future commits.
Signed-off-by: Eric Myhre <hash@exultant.us>

The comment in the ipld.Node type about IsUndefined has been there
quite a while; today we're finally using it.

With this set of changes, the generated code for structs now compiles
successfully again: the Undef and Null thunks are both used there.

Signed-off-by: Eric Myhre <hash@exultant.us>

Fields in these structs are now named.  Embedding is reserved for
things that are very explicitly supposed to carry their methods along.

At some earlier point, I embedded the Type field in some of the
generate structs because it saved me a few keystrokes in refactoring
template strings.  This was not important.

Today I learned: if you have embeddings which cause a name to be
shadowed -- as the generateKindedRejections_String.Type field
could shadow the generateKindString.Type field, for example -- if those
resolve "neutrally" (I don't know if there's a better technical term
for this), then... in that case, the templating system will be like
"yeah, fine, whatev", and it works.  If you give one of those two
fields a *separate type*, the templating system will now reject it as
if no field of that name can be found.

I encountered this while working on the generator for structs (which I
*swear* I will commit any second now, when I stop finding prerequisite
yaks to shave -- there have been many) and trying to use a more
specialized TypeStruct instead of the Type interface.  Whee.

I get the logic of this.  The error message could be better.

Anyway: even though it technically didn't affect generateString (yet),
making the change here for consistency with what's to come
(including specializing the field even though there's currently no
actual need for it).

This isn't a net negative size diff yet, but it certainly will have
that effect momentarily when we add a generator for another kind.
More importantly, it reduces the set of functions in the real
generator file to *just* the relevant ones.

We'll almost certainly extend this to cover the NodeBuilder half of
things as well, later; I just haven't gotten there yet.