easy+: Add a reasonable CI (rustfmt check, verify compilation)

[luciusmagn]

It should be easy to add these, the harder part is making it fast (perhaps by adding sccache or looking into github actions caching options)

• use cargo fmt --check to verify formatting -> run from repo root with the flag --all
• for now, I think only doing a cargo check --all --all-targets is enough to verify the code compiles

In other words, if you commit unformatted code, I will bombard your mom

[lajp]

if you commit unformatted code, I will bombard your mom

Luckily I'm unable to commit unformatted code. Phew!