WIP Check equality under two contexts.
WIP more constraint-based unification
WIP Working on constraint-based elaboration/checking
prototype: Working version of constraint-based elaboration
Purge some warnings
Fix bug in FreeVars.
WIP TwoContexts solver
Fix bug in Elaboration.
Actually add TwoContexts solver...

...and incidentally, new Configuration code
Remove old typecheck code
Add solver files
Change how exceptions work, add a `fatalError' function.