ZTimer clocks: Consider moving to ZST

[chrysn]

ZTimer clocks are currently word-sized, but mutually incompatible because they carry their rate is part of the type anyway. Given that it's highly unlikely that clock using code would be monomorphized on different clocks of the same rate, it might be a good idea to move the clock into something that just has type-level information about the clock.

This would probably change the clock types as they'd gain a new parameter (and there could be a default to make that compatible, but it would be different across the three RIOT provided clocks).

[chrysn]

This would help with #9, as there could then be a Default implementation.