relax rtic-time generics

[burrbull]

[datdenkikniet]

I think this is a pretty good idea.

For clarity, it may be good to rename the TimerQueue to PriorityQueue (since that is what it becomes with this relaxation), and add a pub type TimerQueue<M> = PriorityQueue<<M as Monotonic>::Instant> to indicate what the types' respective goals are, and what they' re bound to. Not sure if it is feasible or any more/less usable in that case, though

[Finomnis]

Might collide with and/or become obsolete through #874.