Don't assemble non-env/bound candidates if projection is rigid
Putting this up for an initial review, it's still missing comments, clean-up, and possibly a tweak to deal with ambiguities in the `BestObligation` folder.
This PR fixes https://github.com/rust-lang/trait-system-refactor-initiative/issues/173. Specifically, we're creating an unnecessary query cycle in normalization by assembling an *impl candidate* even if we know later on during `merge_candidates` that we'll be filtering out that impl candidate.
This PR adjusts the `merge_candidates` to assemble *only* env/bound candidates if we have `TraitGoalProvenVia::ParamEnv | TraitGoalProvenVia::AliasBound`.
I'll leave some thoughts/comments in the code.
r? lcnr
688478fe45