This required changing almost all users of hashmaps to import the hashmap interface first.
The `size` member in the hashmap structure was renamed to `count` to work around a name conflict.
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.
