Implement some tweaks in the new solver
I've been testing the new solver on some small codebases, and these are a few small changes I've needed to make.
The most "controversial" here is implementing `trait_candidate_should_be_dropped_in_favor_of`, which I just implemented to always return false. This surprisingly allows some code to compile, without us having to actually decide on any semantics yet.
r? `@rust-lang/initiative-trait-system-refactor`
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