3944: Look up trait impls by self type r=matklad a=flodiebold
This speeds up inference in analysis-stats by ~30% (even more with the recursive solver).
There's a slight difference in inferred types, which I think comes from pre-existing wrong handling of error types in impls, so I think it's fine.
Co-authored-by: Florian Diebold <flodiebold@gmail.com>
11d400b63b