This reduces some redundancy in the AST data structures and cruft in
the code that works with them. To get a def_id from a node_id, apply
ast::local_def, which adds the local crate_num to the given node_id.
Most code only deals with crate-local node_ids, and won't have to
create def_ids at all.
Most of the fields in an AST item were present in all variants. Things
could be simplified considerably by putting them in the rec rather
than in the variant tags.
Since the decl in a for or for-each loop must always be a local
decl, I changed the AST to express this. Fewer potential match
failures and "the impossible happened" error messages = yay!
Generate appropriate constraints for calls to functions with
preconditions, and reject calls where those constraints don't
hold true in the prestate.
...by which I mean that it works for one test case :-)
Changed function types to include a list of constraints. Added
code for parsing and pretty-printing constraints. This necessitated
splitting pprust into two files (pprust and ppaux) to break a
circulate dependency, as ty_to_str now needs to print out constraints,
which may include literals, but pprust depended on ty.
