Add miri to CI

[DropDemBits]

Fixes #463

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[nikomatsakis]

This is awesome! I see we are getting CI failures though :(

I don't know enough about miri to know if these are known limitations or what -- I imagine maybe we just want to run this on a subset of our test suite?

[DropDemBits]

miri does have support for shimming out to some foreign functions, though not for pipe2 currently. This is an issue for any tests using the insta crate, which eventually ends up calling pipe2.

Thankfully, it's isolated to the older version of salsa, as salsa-2022 doesn't use the insta crate for tests. It's been a bit but I think that's what I did to test out the placement new changes in miri.

The unfortunate thing is that we'd need to list out every package that we'd want to run miri on, but I think that's an acceptable compromise for now.

[DropDemBits]

For now, I made it so that miri only runs on the salsa-2022 packages, but since there is preexisting UB it'll still fail Edit: was from an outdated package, see next comment.

[DropDemBits]

Whoops, turns out locally I had an outdated Cargo.lock file with an older arc-swap. 😅
Though I think we should bump arc-swap to 1.6.0 since 1.5.1 and earlier are known to have UB (see vorner/arc-swap#86).

[DropDemBits]

Added a commit bumping the arc-swap version, though I can split it into a separate PR if need be.

[nikomatsakis]

bors r+

[nikomatsakis]

Thanks, and I appreciate your patience a second time! Things got crazy there.

[DropDemBits]

It's easy to be patient when I'm busy with college again 😆