Better to have the elaborator across multiple files, for ease of
navigation. Also abandon the idea of local unification variables -
we'll keep them global, and try let binding them on leaving a scope.
Also removed 'MV' as a special name type, and added a Meta constructor
in TT which is applied to exactly the right number of arguments for the
environment the meta was constructed in.
It's possible the local unification UCtxt may be more trouble than it's
worth! Instead perhaps we can try let binding unification solutions where
possible on leaving the scope in which they're introduced.
Unification based only on constructor arguments and matching - anything
else fails. Idea is to solve 'easy' local unification problems quickly,
without having to create a global metavar applied to the current context.
