Implement parsing and conversion of generalized where clauses

Going to tentatively open a PR before I move on to other things tonight. I think everything should be good minus a failing test case for region bounds (gonna push that now, i.e 'a : 'b). r? @nikomatsakis 
The only thing not implemented here are `EqPredicate`s so I opened a related issue here: https://github.com/rust-lang/rust/issues/20041.