This checks lambdas where the type isn't immediately a pi binder. First
normalises, if that doesn't work, just guesses and unifies with the
expected type.
Hole list represented as a map now for quicker deletion of solved holes.
Unification returns whether it solved any holes as part of its result,
so we know if it's worth going back to solve constraints if progress has
been made.
Changed 'check' so that it returns the type as a 'Glued', which is a
pair of a term and its normalised form (or, more accurately, the process
to generate each in Core). Sometimes the elaborator needs Term,
sometimes in needs NF; this way we can return both but only actually
generate it when its needed.
So far, mostly done applications, binders and variables. Unification is
still to do.
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.
