8856: Use Chalk for unification r=flodiebold a=flodiebold
- use Chalk's unification, get rid of our own `unify`
- rewrite coercion to not use unification internals and to be more analogous to rustc
- fix various coercion bugs
- rewrite handling of obligations, since the old hacky optimization where we noted when an inference variable changes wasn't possible anymore
- stop trying to deeply resolve types all the time during inference, instead only do it shallowly where necessary
Co-authored-by: Florian Diebold <flodiebold@gmail.com>
edbde25ca2