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- `typed.Node` -> now lives in the `impl/typed` package, more like
other nodes.

- Most of the other essential parts of reasoning about types moved
to `schema` package.  ("typed.Type" seemed like a nasty stutter.)

- `typed.Universe` renamed to `schema.TypeSystem`.

- Current `Validate` method moved to `schema` package, but see new
comments about potential future homes of that code, and its
aspirational relationship to `typed.Node`.

Conspicuously *not* yet refactored or moved in this comment:

- The `typed/declaration` package -- though it will shortly be scrapped
and later reappear as `schema/ast`.  The dream is that it would be
neatest of all if we could generate it by codegen; but we'll see.
(This would seem to imply we'll have to make sufficient exported
methods for creating the `schema.Type` values... while we also want
to make those immutable.  We'll... see.)

- The `typed/gen` package is also untouched in this commit, but
should similarly move shortly.  The codegen really ought to be built
against the `schema.Type` reified interfaces.

Overall, this drops the sheer nesting depths of packages a fair bit,
which seems likely to be a good smell.

This has been deprecated and replaced by the NodeBuilder system
for a good while now; time to scrape it into the dustbin completely.

Tests that were primarily on the mutable node system itself also
drop, so, this is a *very* large delete diff.

A few other tests used MutableNode just incidentally, and those are
quick fixed to use NodeBuilder.
Signed-off-by: Eric Myhre <hash@exultant.us>

All of these struct definitions are horrifying similar in construction
to the same-named definitions in the typesystem package: simply ever
instance of a Type pointer is instead a TypeName string.  Despite being
unattractive, this is correct: since typesystem can have pointers and
cycles, and typedeclaration can't, this is what we end up with.
Signed-off-by: Eric Myhre <hash@exultant.us>

...and all fields are correspondingly unexported.

We're going full immutability on almost all this stuff.  I simply can't
imagine handling and maintaining this any other way.

The typesystem.Type interface also got a few more standard methods.
I'm not sure if having a Universe backpointer is going to be useful
very often or not, but it seems reasonable enough to do; we'll
definitely *have* that information during consistency checking and
reification time, so it should turn out easy to keep it.

The validate_test.go code is now looking a bit janky.  We need
initializers for the typesystem.Type* values.  Those will be the
typedeclaration.* package... which we'll write next.

Note that I skipped any accessor methods on `Type*` objects which are
for representation details.  These might get filled in later, but I'm
gonna see how far I can get without them.  (So far, there's some
uses of the representation details in the validate method, but we've
located that in the typesystem package anyway, so that's... fine.)
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>

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>