Force Rust 1.57 MSRV

[czocher]

This PR forces the CI as well as local dev environments to use Rust 1.57, which is the MSRV for this project.

Closes #94

[czocher]

@CerulanLumina what do we do about the clippy errors for MSRV? My suggestion is to use the stable version for clippy.

[LuminaSapphira]

Clippy on stable should be fine. Any MSRV issues should be caught by the rest of the toolchain, and it seems using an updated clippy should only allow the extra indirection we have in topsort.

Thanks for working this, we appreciate your contributions!

[czocher]

I changed it to validate with stable but also added an allow so that it won't bother people on their local environments.

[Lucretiel]

Thanks for the contribution! I'm worried about forcing all uses of this library to use 1.57, even local development builds (as opposed to using it only as part of a CI matrix). I'll ask the rest of the typeshare team and get back to you.

[czocher]

@Lucretiel I implemented the dev change due to the task mentioned in #94, but if that's not necessary I can remove the local dev part of this PR ;).

[Lucretiel]

I spoke with the team and apparently I was just mistaken, these changes look good! I'm giving it a final lookover then we should be able to merge today

[Lucretiel]

Thanks!