Remove ImpredicativeTypes from the test suite

[RyanGlScott]

A couple of things about the test suite were bugging me, so I decided to patch them up:

• The use of ImpredicativeTypes makes certain aspects of typechecking unpredictable, so let's remove it. This requires wrapping certain rank-n functions in newtypes, but otherwise, it's a straightforward transition.
• Uncomment some tests that were failing to typecheck, and tweak them to avoid impredicative shenanigans.

Along the way, I added a basic .gitignore file (taken from https://github.com/github/gitignore/blob/master/Haskell.gitignore).

[erikd]

Thanks!