Avoid inference guessing in trait solver (e.g., favoring where-clauses over impls)

[nikomatsakis]

The current trait solver performs "inference guessing" in various places, leading to incompleteness (failure to find solutions that exist). We avoid unsoundness by eliminating incompletness in coherence. But it may be possible to refactor the trait solver to instead return "guidance"; it would be up to HIR type check or other phases to decide if/when to apply this guidance. This would lead to more programs being compiled but also a cleaner overall setup. Experimentation is required to see if this truly works.

cc rust-lang/rust#41756

[lcnr]

update: not actively worked on, requires the solver to be more settled, currently not a main focus.