Add clippy to build action.

[kinetiknz]

Inspired by #186, updated for GitHub Actions.

Running a separate step for nightly clippy is annoying, but non-nightly builds don't recognize clippy::upper_case_acronyms and as far as I understand it's not currently possible to disable lints via Cargo.toml or clippy.toml.

[baumanj]

Generally looks good, but I'd like to understand the rationale around the To -> Into naming change before signing off.

[kinetiknz]

I removed the nightly-only clippy CI step in favour of handling lint allow lines in the code itself - this has the advantage of in the generating the same results locally for a simple cargo clippy without having to copy the project-specific settings out of the CI config.

[baumanj]

Looks good! Can we just add these comments referencing the appropriate clippy issue?