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...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>