time 2.0: stepable, non monotonic,

[mockersf]

Objective

• Time falls a bit short of some scenarios:
  • It's backed by a Duration, it can be hard to use as a number of steps since beginning
  • It's monotonic, so travelling backward in time for replay is not possible

Solution

• Make Time generic over the type used to back the impl
• Add new contexts that can travel back in time

[alice-i-cecile]

Broadly in favor of this design, and the feature itself. This would be useful for simulation. Needs more docs, examples and a quick release note before I can approve or give a full review though.