Auto merge of #50937 - nikomatsakis:chalkify-engine-2, r=scalexm

implement the chalk-engine traits

Preliminary implementation for the Chalk traits in rustc. Lots of `panic!()` placeholders to be filled in later.

This is currently blocked on us landing https://github.com/rust-lang-nursery/chalk/pull/131  in chalk and issuing a new release, which should occur later today.

r? @scalexm
cc @leodasvacas