replace custom wf check with well_formed constructor

[shua]

This change is mostly motivated by a comment on prove_where_clause_well_formed:

FIXME(oli-obk): figure out why is this a function and not a judgment_fn.

I originally started with converting it to a judgement_fn but found I was duplicating a lot of logic from prove_wc which already exists. So I settled on moving the logic to a WhereClause::well_formed method which creates a set of goals to prove, and then calls prove_wc on those goals.

I'm not certain this is an improvement on the current code. There is more code being run (constructing a set of goals, then checking them is more than just checking), so I wouldn't expect any performance improvement, and it sort of looks like the same code just in a different place, so it's not removing much. The only justification I have is that I wouldn't ever do the reverse of this change (remove the well_formed constructor in favour of custom well_formed check).

[nikomatsakis]

I think it's good. I've been a bit reluctant to add "synthetic" predices if we can avoid it, but this feels like one we are going to want.