Rollup merge of rust-lang#56214 - scalexm:unification, r=nikomatsakis

Implement chalk unification routines

`ResolventOps` and `AggregateOps` are mostly straightforwardly translated from chalk. I had caught a few bugs already in my `chalk` branch and backported fixes to this branch, but there may be other ones left. EDIT: I hope there are none left now :)

Fixes rust-lang#54935.

Author: kennytm