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 
I also created a follow up issue (here)[https://github.com/rust-lang/rust/issues/20049], and marked the code appropriately.