tor-rtcompat: Introduce CoarseTimeProvider, a new part of Runtime
This is a breaking change for people who implement Runtime, but not for anyone else.
This is related to #496, which discusses panics in coarsetime (on which point see #496 (comment 3011692)).
Some form of fast-to-query time API s a prerequisite for the queue memory tracking (!1997 (merged)).
Having seen how this MR came out, I'm comfortable with having this as part of the Runtime API and think we're not boxing ourselves into any unfortunate corners.
Assigning review, since I think Nick has opinions and most of the background.