Move MetaVars to AST, and expand AST polymorphism
Add 'colors' lens for main options
(-->) in AST for nondependent function types
Simpler definition for _Id
parse x->y
Add metavariables to parser
New eval function in Main

addEnv has vanished for now though
Swap arguments to Cxt.Def

Now `Def e t` is same order as `e: t`
Make Unify.solve a toplevel function
Use MetaExps in reductions and type checker
Bring tests up to date