Fix new rust & clippy warnings; Simplify CI

[hasezoey]

This PR fixes new warnings by rust and clippy and also simplifies the CI workflow by:

• combining build & test jobs
• switching to Swatinem/rust-cache for our caching needs
• directly install components instead of another step
• test MSRV in addition to stable
• enable workflow run on any branch

Finally, set a MSRV as listed on lib.rs (older may work, but didnt test)

[longsleep]

Lgtm - with the bump of the Rust version to 1.70 maybe the next release should reflect that by going to 0.2.0.

[hasezoey]

with the bump of the Rust version to 1.70

I dont think we advertised any rust version before.

maybe the next release should reflect that by going to 0.2.0.

Along with some other changes, we can make it a 02.0

[hasezoey]

Accidentally closed, pushed the wrong button instead of "Comment".

with the bump of the Rust version to 1.70 maybe the next release should reflect that by going to 0.2.0.

I dont think we advertised a rust version before and i didnt test any lower version, but along with some other changes this will be 0.2.0.