Remove dependency on arrayvec

[giarc3]

Would need to bump MSRV to 1.48 because of this addition to Rust. We could possibly make do with an implementation available start in 1.43, but it doesn't seem as clean.

Open to opinions on if the bump to 1.48 is acceptable, or if we should make an attempt at the 1.43 version instead.

CI changes:

• More resilient way of installing rustfmt, as it's not always available on nightly.
• Switch to using rust_cache, which we've been moving towards already as we've been adding sccache. This is just an intermediate step towards that with less noise and some bugfixes built in

[clintfred]

I think the PR description should mention the build changes. I think you are just using rust_cache?

[clintfred]

We need a minor patch version bump and an updated changelog.

[giarc3]

I think the PR description should mention the build changes. I think you are just using rust_cache?

I added a brief description of the CI changes. They were fairly minor.

[skeet70]

I think if we're bumping MSRV we should at least minor bump. Too easy for a patch version to break builds. We might want to make public policy about that.

[giarc3]

I think if we're bumping MSRV we should at least minor bump. Too easy for a patch version to break builds. We might want to make public policy about that.

You're 100% right. We do need a policy on that because we've been burned by it with our dependencies multiple times