Some `compare_method` tweaks
1. Make some of the comparison functions' names more regular
2. Reduce pub scope of some of the things in `compare_method`
~3. Remove some unnecessary opaque type handling code -- `InferCtxt` already is in a mode that doesn't define opaque types~
* moved to a different PR
4. Bubble up `ErrorGuaranteed` for region constraint errors in `compare_method` - Improves a redundant error message in one unit test.
5. Move the `compare_method` module to have a more general name, since it's more like `compare_impl_item` :)
6. Rename `collect_trait_impl_trait_tys`
83a28ef095