Auto merge of #115996 - lcnr:intercrate_ambiguity_causes-uwu, r=compiler-errors

implement `intercrate_ambiguity_causes` in the new solver

I added some comments but this is still somewhat of a mess. I think we should for the most part be able to treat all of this as a black box, so I can accept that this code isn't too great.

I also believe that some of the weirdness here is unavoidable, as proof trees - and their visitor - hide semantically relevant information, so they cannot perfectly represent the actual solver behavior.

There are some known bugs here when testing with `./x.py test tests/ui --bless -- --target-rustcflags -Ztrait-solver=next-coherence`. While I haven't diagnosed them all in detail I believe we are able to handle them all separately

- `structurally_normalize` currently does not normalize opaque types, resulting in divergence between the solver internal `trait_ref_is_knowable` and the one when computing intercrate ambiguity causes.
- we don't add an `intercrate_ambiguity_cause` for reserved impls
- we should `deeply_normalize` the trait ref before printing it, that requires a "best effort" version of `deeply_normalize`

r? `@compiler-errors`

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