Make `Impl.trait_` a `Path`, not a `Type`

Make `Impl.trait_` a `Path`, not a `Type`

It should only ever be a `ResolvedPath`, so this (a) enforces that, and
(b) reduces the size of `Impl`.

I had to update a test because the order of the rendered auto trait impl
bounds changed. I think the order changed because rustdoc sorts auto
trait bounds using their `Debug` output.