Allow writing self.f within a class that has a field f. Currently,
the compiler accepts either self.f or f. In a future commit I'll
require writing self.f and not f.
Not sure whether self.f() works if f is a method (making sure that
works next).
Programs using classes with fields only (no methods) compile and run,
as long as nothing refers to a class in a different crate (todo).
Also changed the AST representation of classes to have a separate
record for constructor info (instead of inlining the fields in the
item_class node), and fixed up spans and pretty-printing for
classes.
Class tests aren't working yet, but they fail a little later :-)
Also, make the parser correctly set a constructor's result type to
its enclosing class type.
