Auto merge of #37100 - dikaiosune:master, r=eddyb

Change Substs to type alias for Slice<Kind> for interning

This changes the definition of `librustc::ty::subst::Substs` to be a type alias to `Slice<Kind>`. `Substs` was already interned, but can now make use of the efficient `PartialEq` and `Hash` impls on `librustc::ty::Slice`.

I'm working on collecting some timing data for this, will update when it's done.

I chose to leave the impls on `Substs<'tcx>` even though it's now just a type alias to `Slice<Kind<'tcx>>` because it has the smallest footprint on other portions of the compiler which depend on its API. It turns out to be a pretty huge diff if you change where Substs's methods live :smile:. That said, I'm not necessarily sure it's the *best* implementation but it's probably the easiest/smallest to review.

Many thanks to @eddyb for both suggesting this as a project for learning more about the compiler, and the tireless ~~handholding~~ mentorship he provided.

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