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ipldfree.Node is now a much better implementation of Node.

In particular, this means it will work correctly when combined with the
typed.Node wrapper implementations which expect to be able to use
ErrNotExists for logic purposes.
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>

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>

Leaving as a comment for now because it's not at the top of the
priority-heap of refactors I want to chase right now.
Signed-off-by: Eric Myhre <hash@exultant.us>

A bit fun; wish there was a way to compress that block of ifdefs about
optional vs nullable stuff, but so far nothing clearer has emerged.

Added ErrIteratorOverread while at it.  Should refactoring other node
types to use it too, I suppose.  But perhaps we'll just accumlate those
todos for a while, since there's already one other error refactor with
a chain of blocks in front of it.

The latter isn't used yet; that's not the main goal of this branch (and
I think I'd rather do the refactor to use ipld.ErrNotExists after
moving the PathSegment types from the selector package into the root...
which in turn I'm not going to do while some of the other current work
on Selectors is in progress in other branches.  So!  Latur.); I just
wanted it around for documentation and clarity of intent.

This also adds a couple of other dummy references to the 'typed'
package.  I know keeping a set of packages-used-in-this-file is a
common step in the development of codegen systems targetting golang,
but I'm going to try to avoid it until the need is really forced.

typed.Node.Representation(), which returns another Node, should address
most of the infelicies we've found so far in trying to plan nice code
that works over the schema layer.

Also added in this comment: ipld.ReprKindSet, primarily for use in the
ErrWrongKind error.  It comes up often enough we might as well formalize
the thing.

It's high time we got started on well-typed errors; and this type name
is already hinted at by the codegen outputs, so let's make it real.

This file has been hanging around on my disk uncommitted for a fair
while, and some of the comments about review-needed are stale, but I'm
keepin' em for historical interest.  I think I have fixes planned that
will straight out all of them.