Implement some FIXME methods in the new trait solver
Implement just enough of the solver's response logic to make it not ICE.
Also, fix a bug with `no_bound_vars` call failing due to canonical bound vars.
r? `@lcnr`
Check `impl`'s `where` clauses in `consider_impl_candidate` in experimental solver
Check impl's nested predicates as part of the recursive evaluate in `consider_impl_candidate`.
<sub>Unless, for some reason, these are intentionally **not** checked here -- in which case, I really don't understand where they're being checked...<sub>
r? ```@lcnr```
