Reschedule timer in case of early wakeup even if a valid waker exists

[fko-kuptec]

This pull request is my first attempt at fixing #7.

A downside to this way of fixing it is, that polling a future would cause many redundant wakers to get scheduled. But I don't know if there is any way to avoid this with the current API of embassy-time-driver.

[ivmarkov]

Thank you for your contribution!
I'm on travel ATM and will be able to look at it mid to end of next week.

But yeah, the problem does not have a trivial solution, as you found out. The major roadblock for an easier mapping of the async-io Timer API to embassy-time is that the Future trait is implemented directly on async_io::Timer which is at odds with the embassy-time API, where the Timer type has an explicit Timer::wait method which returns a sub-future. Hence all hacks in the code.