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As detailed in comments a few commits ago, this is part of a big, big
roll towards keeping linking details far enough off to one side that
one can actually use most of the IPLD system without forming an
explicit compile-time dependency on any linking features (until, of
course, one uses the linking features).

This is a surprisingly small diff, because... well, because most of
the *interesting* features around linking simply weren't implemented
yet, and at this point everything that is has already been isolated
in the new cidlink and related encoding packages.
"CID" was *already* just a semantic placeholder that meant "eh, link".
Signed-off-by: Eric Myhre <hash@exultant.us>

An "immediate" rather than generative function is also available;
the docs contain caveats.

At the moment, these distinctions are a bit forced, but we want to be
ready for when we start getting truly huge maps where the generative
usage actually *matters* for either memory reasons, latency reasons,
or both.

Separated Length rather than trying to pull double-duty the Keys
method; the previous combination was just utterly silly.

The implementations in the bind package are stubs; that package is
going to take a lot of work, and so the focus is going to be entirely
on keeping the 'free' package viable; and we'll revisit bind and such
when there's more dev time budget.
Signed-off-by: Eric Myhre <hash@exultant.us>

ipldfree and ipldbound and fluent and all the interfaces now have all
the isNull methods, asBytes, setBytes, etc.  (Previously this was a
little swiss-cheesy.)

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

And ReprKind moves from the typed package to the ipld main package.
It's hard to get too much done without the standardization of ReprKind.

Between the Kind() and Keys() methods, it should now be possible to
perform traversals of unknown nodes.

This diff just worries about implementing all the Kind() methods.
Keys() has some additional questions to handle (namely, map ordering?).
Signed-off-by: Eric Myhre <hash@exultant.us>

And a test of building an array and pulling a value back out.
Signed-off-by: Eric Myhre <hash@exultant.us>

(Mostly; all the scalars work; the composites are todo's.)

Heading in the direction of being able to construct stuff for testing.
Almost there.
Signed-off-by: Eric Myhre <hash@exultant.us>