traits/fulfill: allow `stalled_on` to track `ty::Const::Infer(_)` (unused yet).
This PR addresses the representation side of #70180, but only *actually collects* `ty::Infer`s via `Ty::walk` into `stalled_on` (collecting `ty::ConstKind::Infer`s requires #70164).
However, it should be enough to handle #70107's needs (WF obligations are stalled only on the outermost type/const being an inference variable, no `walk`-ing is involved).
This is my second attempt, see #70181 for the previous one, which unacceptably regressed perf.
r? @nikomatsakis cc @nnethercote
7bd86cee49