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 
Oh, yes, I actually think this id is kind of ... superfluous? Let me re-investigate. I remember wondering what purpose it served.