All field or method references within a class must begin with "self." now.
A bare reference to a field or method in the same class will no longer
typecheck.
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).
Added class support to the parser, prettyprinter, fold, and visit.
(See Issue 1726.)
This is WIP -- the test case is xfailed, and attempting to compile
it will error out in resolve.
