[$] Rust's next-generation trait solver

Rust's compiler team has been working on a long-term project to rewrite the trait solver — the part of the compiler that determines which concrete function should be called when a programmer uses a trait method that is implemented for multiple types. The rewrite is intended to simplify future changes to the trait system, fix a handful of tricky soundness bugs, and provide faster compile times. It's also nearly finished, with a relatively small number of remaining blocking bugs.