Refactoring in support of higher-ranked traitbounds

Various miscellaneous changes pushing towards HRTB support:

1. Update parser and adjust ast to support `for<'a,'b>` syntax, both in closures and trait bounds. Warn on the old syntax (not error, for stage0).
2. Refactor TyTrait representation to include a TraitRef.

r? @pcwalton 
nit: weird indentation