Add contexts that use FakeClock rather than the system time.

[DPJacques]

These contexts work similar to the standard library's context.WithTimeout and context.WithDeadline, expect they use a clockwork.FakeClock for deadline determination.

By nature, the generated contexts ignore parent cancelation if the parent's cancelation returned conext.DeadlineExceeded. However, an unfortunate but necessary nuances is that once the parent is cancelled with conext.DeadlineExceeded, future calls to cancel the parent due to otherwise normal calling of the parent's cancel function are not communicated to the child. As far as I can tell, there is no way to cleanly, reliably communicate that.

Once this occurs, and the child has ignored the parent cancellation, the only way to cancel the child is to advance its fake clock past the context's deadline or call the cancel function returned when the context was created.

Other changes that were made in support of this change or as targets of opportunity:

• newFakeTimer was made a standalone function since it needed to be used in 2 places.
• Functions with the pattern new*AtTime were introduced to ensure the FakeClock mutex is held when we need to call time.Time.Sub(FakeClock.Now()) and pass that to a function that also needs to hold the FakeClock mutex. The common use case for this is supporting functions that take an absolute time rather than a duration.
• Use slices.DeleteFunc where we can.

[3vilhamster]

API is nice, and it fits into my initial request!
I am looking forward to using it inside the cadence-workflow.

[DPJacques]

Merged because there are a few other clean ups and fixes I'd like to do before bumping the version.