Ambiguous use of `Time::Span` for duration and monotonic clock

[straight-shoota]

I split off the discussion thread about sleep with a wall clock. It’s independent of the type for monotonic clock readings.

These two partial quotes are relevant for this thread, though:

@ysbaddaden

Maybe there could be a Time::Instant type to represent a point in time? Either an abstract struct or a module, so specific clocks (Time::Monotonic, Time::Realtime, …) could inherit most of their implementation, since time operations (add, sub, <=>, to_f, …) don’t depend on the clock.

@straight-shoota

A generalization might be a good idea. I’m not quite sure how useful it would be because the different clocks generally serve very different purposes.
Time::Instant should work as a name for that.