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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.

- `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.