Allow class methods to have type parameters (this is a change from the
original classes proposal).
Add test cases for classes with type parameters, and classes with methods
that have their own type parameters.
1. Enforce mutability declarations on class fields. Don't allow any
mutation of class fields not declared as mutable (except inside the
constructor).
2. Handle classes correctly in shape (treat classes like records).
This will make it easier to convert crate_ctxt into a region pointer, since
there are functions that return crate contexts. There would be no way to type
these functions if crate_ctxt had to be an inferred region pointer.
