Implement some candidates for the new solver (redux)

[compiler-errors]

Based on #106718, so the diff is hard to read without it. See here for an easier view until that one lands.

Of note:

• 44af916020fb43c12070125c45b6dee4ec303bbc fixes a bug where we need to make the query response inside of a probe, or else we make no inference progress (I think)
• 50daad5acd2f163d03e7ffab942534f09bc36e2e implements consider_assumption for traits and predicates. I'm not sure if using sup here is necessary or if eq is fine.
• We decided that all of the instantiate_constituent_tys_for_* functions are verbose but ok, since they need to be exhaustive and the logic between each of them is not similar enough, right?

r? @lcnr