Initial big bump in Elm progress

Lots of small hard-to-figure out changes!

New features

Add two garnet commands: rerun-benchmarks (re-run a suite) and refactor (run a single example from stdin)
In type checking, unknown constructors are now assumed to be "polymorphic variants" and of a single fixed, universal type
Check type consistency (roughly, whether the types are the same "shape") in Unification rather than exact equality to better handle polymorphism
Beef up Main, including by abstracting over free variables in the input snippet so that they can be assigned types
Add make regen-stdlib command

Ignore module prefixes when parsing elm-format output so that our types line up with the actual Elm types
Fix bug in Unification_adapter that did not translate hole names correctly
Fixed a type checking bug where constructor type arguments were not properly instantiated in the context of catamorphisms