Improve the type checker.

Make instance checks work correctly (skolemization) and add a slew of tests related to instance checking.
Make annotated order-n functions rank-n in row variables. Scalar variables are still restricted to prenex polymorphism.
Rescope quantifiers in generalized function types. This allows the limited form of higher-rank polymorphism to be inferred.
Generalization was only using the free variables of a type; remove this hack by fixing a bug in enumerating the bound variables of a type environment.
Remove the useless id function.

evincarofautumn / kitten