Implement embedded_hal_timer::Timer

[jannic]

Open questions:

• Naming
• Should we feature-gate this behind some unstable feature for now?
• This doesn't detect overflows of the u64 tick counter. Most likely not an issue in practice, for microsecond ticks. But it should at least be documented.
• Needs a MSRV bump because embedded-hal-timer requires edition2024. (See failing CI)

Before merging this, it should also be implemented for rp235x-hal.