Add CI

[kupiakos]

Ensure that at the very least:

• cargo test works
• cargo +nightly test --all-features works

It could also:

• Check the MSRV
• Check formatting
• Check clippy

[danielhenrymantilla]

Yes, this is so key; I guess back in the day I got lazy and was just cargo testing myself 😱.

This should be top-priority before anything else (at least the testing part; the extra features such as checking formatting would have to be embedded within their own "format properly the codebase" efforts