stm32: use typelevel timer type

[xoviat]

Reduce code duplication and allow use of dma for 32 bit timers. This precludes type-erasure of 32 bit timers, but not type erasure in general (eg. we would have Timer<U16> and Timer<U32> if we type-erased the timers).

A few points on this:

• For the low-level timers, this requires passing in the correct word to arr, ccr, and any dma buffer. This is in contrast before when u32 was accepted inconsistently.
• For the high-level timer, only the dma buffer is leaked; the other functions accept u32 for both 16 and 32 bit timers. This is because there is no way to convert a 16-bit buffer to a 32-bit buffer. It could be possible to abstract even this away, but I do not think this is ever a use-case that is needed for the additional complexity.