Rollup merge of #147840 - jdonszelmann:unsizing-coercions, r=lcnr

Rework unsizing coercions in the new solver

Replaces rust-lang/rust#141926, contains:

- a commit adding tests that fail before this work
- the two commits from the previous PR
- a commit in which these tests are fixed
- finally, a fixup for an in my opinion rather large regression in diagnostics. It's still not perfect, but better?

I hope this is roughly what you had in mind

Fixes rust-lang/trait-system-refactor-initiative#241 and rust-lang/trait-system-refactor-initiative#238, adding tests for both

r? ````@lcnr````

Author: Zalathar