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.
