Reintegrate chalk into rustc

[jackh726]

TL;DR

Reintegrate chalk into rustc using chalk-solve under the -Z chalk flag.

Links and Details

This is working towards the eventual™ goal of using Chalk as the rustc trait solver. There used to be an experimental implementation of Chalk in rustc using chalk-engine. It became outdated and was removed in rust-lang/rust#69247. Since that first experimental implementation was added, Chalk has gone through lots of changes, including adding a new crate, chalk-solve, which is supposed to implement Rust logic. As such, with this newer implementation, the interface between rustc and Chalk is a bit higher-level.

The PR for this initial MVP implementation is here: rust-lang/rust#69406

Mentors or Reviewers

c.c. @nikomatsakis
c.c. @rust-lang/wg-traits

[nikomatsakis]

@rustbot second

I'm going to exercise the "I consider this unlikely to be controversial clause" and r+ rust-lang/rust#69406 before it bitrots...we can always back it out. (Besides, if I know bors, it'll take 2 weeks to land anyway 😛)

[nikomatsakis]

Closing, FCP expired. =)