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

See previous commit for more discussion of the child packages to come,
or the schema.md file in the docs dir, which describes similar.

Several large todos in comments.

Quite importantly: typed.Node is now an **interface**, not a concrete
struct itself.  The godoc comment should explain this nicely; long
story short, this is going to be relevant when we get to codegen and
other advanced form of native integration.
Signed-off-by: Eric Myhre <hash@exultant.us>

I need there to be separate packages for:

- the typed.Node interface, and any top level helpers;
- the typesystem implementation
- the typesystem *declarations* (distinct because it's what you
 construct when parsing the ast -- doens't contain pointers yet)
- and (eventually) a parser and a fmt package for the ts declarations.

And right now all those are heaped together.

Not sure where the validate method -- the thing that's actually touched
in this diff -- will end up.  Either the top level, or the typesystem
implementation package, probably.
Signed-off-by: Eric Myhre <hash@exultant.us>

This will be for the active path -- if we also follow through on the
idea of having a just-in-time checked Node wrapper, a lot of these
checks might end up duplicated.  We'll DRY that up when we get there.

Doing accumulation of errors.  Might get loud.  We can come back and
and early-halt parameters or other accumulation strategies later.

Added IsNull predicate to core Node interface.

Going to want enumerated error categories here for sure, but punting
on that until we get more examples of what all the errors *are*; then
I'll come back and tear all this 'fmt.Errorf' nonsense back out.
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>