Add support for associativity to the monadic pretty printer.
Add rigid type variables that cannot be unified so generally.

These will be used in type annotations.
Add syntax for type annotations.
Add parser support for type annotations.
Add support for type annotations to the type-checker and renamer.
Remove an unused test.
Fix a failing test case.
Uncomment some tests.
Add support for =>* with same arguments but different return types.
Merge branch 'feature/type-annotations'