Replace ty_param_bound with trait_ref in AST

[brson]

These two things serve the same purpose - to name a trait as a constraint - but ty_param_bound is just a typedef for Ty. It's more complex than necessary.

[pnkfelix]

Not critical for 0.6; de-milestoning

[graydon]

This is no longer true, ast::TyParamBound names either a trait or a region. Closing.