handling overflow in fulfill

[lcnr]

rust-lang/rust#121322 changed overflow in fulfillment to simply return, treating the overflowing goals as ambiguous and reevaluating the overflowing goals in the next call to select_where_possible. This does not impact coherence as it only uses a single call to select_where_possible.

Outside of coherence this can likely result in hangs and I believe we generally want some special diagnostics in case there's some overflow hitting the recursion limit. We may even want to fatally error when hitting them or something 🤷