Fix all clippy lints

[jbeaurivage]

Summary

Fix all HAL clippy lints.

Also up for debate: I've switched the CI build step to clippy, and having it deny all clippy warnings. However some might think that it is too aggressive.

Closes #574.

Checklist

• CHANGELOG.md for the BSP or HAL updated
• All new or modified code is well documented, especially public items
• No new warnings or clippy suggestions have been introduced (see CI or check locally)

[ianrrees]

I like the idea of requiring a clean Clippy pass in CI - it seems like overall there's less headache in preventing the warnings at the outset, vs everyone ignoring them in later development

[jbeaurivage]

Sounds good. That doesn't mean we should blindly follow clippy's advice all the time; #[allow]ing certain lints is totally acceptable, but I think we should try to explain why we're allowing it with a comment where reasonable.

[ianrrees]

Showing my ignorance here: does clippy also build the code? Just wondering if there might be some kinds of issue that would pass cargo clippy but not cargo build - if so we might want CI to do both

[jbeaurivage]

@ianrrees you're right, it doesn't build the code - sometimes things like linker errors will pop up when building but not checking. However, since we do build the BSP examples, I'm fairly confident these issues would be caught by CI regardless.

[ianrrees]

since we do build the BSP examples, I'm fairly confident these issues would be caught by CI regardless.

Yep, that makes a lot of sense to me! Any issues that "escaped" would be good to cover with additional examples - sortof a build time regression test suite.