rustc: Unify variables using a single type instead of keeping a list and doing it all at the end. This will give us better error messages.

And fix a the single screw-up in the source code it caught.

No functional change. Largely removed the native module/non-native module distinction from resolve.rs

and kill dead code.

and various other tidying in typestate

Added visit_fn_pre and visit_fn_post fields that get applied to
both regular functions and methods.

This required quite a bit of tiresome plumbing about of spans.
On the bright side, now other errors can be converted to span_err too.

Includes test cases.

Started by rewriting collect_locals to use walk instead of fold

(Also, feel free to start adding logging to your code. Really. It's fast.)

(And remove a boatload of duplicate imports from the typestate files.)

Name collision checking in local scopes will follow soon.