This PR implements support for trait objects on the formdyn Trait.

The implementation works as follows:

1. Ifdyn Trait appears somewhere as a declared type then we create aDynTraitType that consists of the traitTrait.
2. Thedyn Trait has a type parameter for every type parameter ofTrait. There is a 1-to-1 correspondence but they're not equal.
3. Thedyn Trait type is made to implementTrait as if there was animpl<A, B, ...> Trait<A, B, ...> for dyn Trait<A, B, ..> block in the source.

Currently the PR doesn't account for theauto traits that one can specify with a+, for instancedyn Clone + Send + Sync. I don't know if we have a need to consider those, but if we do it should be possible by building on top of the work in this PR.

DCA shows a 0.33% point increase in resolved call targets and no change in analysis time.