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.
This change uses the same code for handling the "self" reference for
classes as is already used for impls/ifaces. This allows removing the
extra maybe_self_id argument (which was just for classes) to trans_closure
that I added before. I also rewrote the translation for class ctors so
that it doesn't generate new AST nodes (instead translating directly).
Also changed visit so that it visits class ctors correctly with visit_fn,
and changed typestate to not do return-checking when visiting a class ctor.
Cross-crate method calls don't work yet. Added
run-pass/class-method-cross-crate to test that, but it's xfailed
References to fields within methods don't work yet. Added
run-pass/class-methods to test that, but it's also xfailed
