This will make it easier to convert crate_ctxt into a region pointer, since
there are functions that return crate contexts. There would be no way to type
these functions if crate_ctxt had to be an inferred region pointer.
- Move io, run and rand to core.
- Remove incorrect ctypes module (use libc).
- Remove os-specific modules for os and fs.
- Split fs between core::path and core::os.
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.
