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.

[thejpster]

2^64 microseconds is 584 millenia (or 584,542 years). I think we're fine to claim that overflows are impossible, and ignore the -1 here.

[thejpster]

Does it need to own the timer? It seems like it would be OK to make a bunch of these, and have them unsafely conjure up the timer for reading whenever required? Or just have a freestanding function called fn get_time() -> u64 that all these types can use?