🚀 create a query for the program clauses needed to solve a given goal

[nikomatsakis]

One of the key parts of the interface between the Chalk Solver and rustc is when the solver requests the program clauses to solve a given goal:

    /// Returns the set of program clauses that might apply to
    /// `goal`. (This set can be over-approximated, naturally.)
    fn program_clauses(
        &self,
        environment: &I::Environment,
        goal: &I::DomainGoal,
    ) -> Vec<I::ProgramClause>;

In Rustc, this is our chance to do some queries and figure out the full set of clauses. This is building on the work in #49177, where we create the lowering rules for a trait, item, etc (see also the rustc guide coverage).

The set of clauses is somewhat specific to the domain goal:

• Implemented(P0: Trait<...>)
  • come from the header of Trait
  • come from each impl
• FromEnv(P0: Trait<..>)
  • these come -- ultimately -- from the param environment
  • but because of implied bounds rules we need to combine with the rules from traits
  • we need to compute transitive closure of traits reachable from the env and combine their program clauses, or else iterate over "all traits" (which is actually not easy)
  • I have some code working towards this on my WIP branch for this issue
• ... the rest have to be written out :)

I'll probably keep iterating on this, but if somebody is keen to hack on it, I'd love to collaborate.

[scalexm]

Implemented in #55203 and #55649.