Merge #1262

1262: Where clauses and other Chalk improvements r=matklad a=flodiebold

This adds support for where clauses to the Chalk integration; it also adds FnDef lowering and partly handles auto traits.

One thing I'm not sure about is the error handling -- what do we do if we can't
resolve a trait reference in a where clause? For impls, I think it's clear we
need to disregard the impl for trait solving. I've solved this for now by
introducing an 'unknown trait' that has no impls, so if we encounter an unknown
trait we can use that and basically get a where clause that's always false. (The
alternative would be somehow not returning the impl to Chalk at all, but we
would need to know that we need to do that in `impls_for_trait` already, and we
don't resolve anything there.)

A bit surprisingly, this has almost no impact on the type inference stats for RA, probably because of missing edge cases. Probably impl Trait support and closure support will do more.

Co-authored-by: Florian Diebold <[email protected]>

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