Add clippy and rustfmt to Rust build

[samstokes]

As discussed in #420 this adds clippy (style checks) and rustfmt (code formatting check) to the Rust build.

Details:

• following OCamlformat all of the things! #477 this checks formatting only in CI, not in dev builds. cargo fmt is actually very quick (sub-second on my machine) so it'd be entirely feasible to run it in dev too, but the dev use case is better served by editor integration, and anyway for quick feedback I'd rather see if my code compiles and passes tests before fixing possible formatting errors.
• this does run clippy checks in dev (after tests), but doesn't fail the build for warnings. In CI, warnings do fail the build.
• for some reason clippy's default behaviour is to style-check every single crate that your code depends on before actually checking the code you wrote. I can't work out how to turn that off. On the plus side I think build caching should mean it only does that occasionally.

N.B. this PR is based on #478 (for the sake of having some non-trivial code to check) - once that merges I'll rebase this branch and change the base of this PR to master.

[samstokes]

The CI failure is in the OCaml build, and I think is because #478 needs rebasing onto master - hoping the rebase will fix both builds.

[IanConnolly]

or some reason clippy's default behaviour is to style-check every single crate that your code depends
on before actually checking the code you wrote. I can't work out how to turn that off. On the plus side
I think build caching should mean it only does that occasionally.`

IIRC it's not actually checking them, but it does need to build them as far as the HIR stage to get type information to be able to lint the stroller code.

[IanConnolly]

This looks good to me! It can land whenever the other PR gets over the line

[pbiggar]

Apologies for closing this, was unintentional, no idea how it happened.