Timer V2 API (for thumbv7)

[glaeqen]

Summary

Old API for timers was quite bare-bone. New one tries to expose more functionality in a typesafe and ergonomic way.

New abstraction is not complete in terms of what underlying peripheral has to offer, eg. event counting or capture channels are not exposed. However it should be possible to extend it in the future.

thumbv6m implementation was omitted because

• it lacks clock::v2 support for now
• current implementation of traits assumes availability of timers in "even-odd" pairs, which makes it not exactly trivial to port.

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)
• unsafe documented
• TODOs resolved