This makes it possible to move them between crates without confusion,
and to instantiate them at a point where the monomorphizing
substitutions are known.
Issue #1944
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.
