Misc minor improvements and updates

[est31]

Best reviewed by commit. Improvements:

• use edition 2021
• turn the repository into a workspace, yields a shared Cargo.lock and target directory
• use docs.rs for the docs link
• add github actions CI
• address clippy lints, including a correctness one

[oli-obk]

Oh wow I haven't looked at this crate in years. Thanks so much!