Setup GitHub actions for CI

[CosmicHorrorDev]

This sets up and action that runs on:

• Pushes
• Pull requests
• A weekly schedule (to proactively point out any new warnings from new stable/beta releases)

This enforces:

• cargo fmt, cargo clippy, cargo test, and cargo doc warnings and hard errors on stable and beta
• cargo check and cargo test hard errors on an MSRV version
• cargo miri test on nightly

[laurmaedje]

For now, I would prefer to remove clippy and MSRV completely. Maybe I'll give clippy another try and see about it then. :)

[CosmicHorrorDev]

Done and done

[CosmicHorrorDev]

Notably I switched cargo clippy -- -D warnings to a cargo check -- -D warnings as a heads up

[laurmaedje]

Thanks for all your pull requests!

[CosmicHorrorDev]

Thanks for the library! I hope to use it in the future :D

[CosmicHorrorDev]

Forgot that cargo check can't be passed Rust flags. I'll throw up a PR to fix that

[laurmaedje]

Doesn't cargo check get executed anyway through cargo test?

[CosmicHorrorDev]

I don't normally deny warnings on cargo test because I'm less concerned about warnings that crop up there, but I try to keep cargo check output clean, so that it's easy to notice any bug-prone warnings on future incompat notices