Simplify `process_obligation`.

Simplify `process_obligation`.

`process_predicates` returns a
`Result<Option<Vec<PredicateObligation>>>`. `process_obligation` calls
it and then fiddles with the output (using `map`, `map`, `into_iter`,
`collect`) to produce a a
`Result<Option<Vec<PendingPredicateObligation>>>`.

This function is sufficiently hot that the fiddling is expensive. It's
much better for `process_predicate` to directly return a
`Result<Option<Vec<PendingPredicateObligation>>>` because `Ok(None)`
accounts for ~90% of the results, and `Ok(vec![])` accounts for another
~5%.