Various small fixes

[ianrrees]

Summary

Most substantial change here is probably the implementation of ehal 0.2 CountDown timers, which is what broke the neopixel demos.

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 - CI will deny clippy warnings by default! You may #[allow] certain lints where reasonable, but ideally justify those with a short comment.

[ianrrees]

This resolves #748

[jbeaurivage]

Thanks for all the maintenance work you've been doing recently @ianrrees!