Provide NonConformance's wait_time_from method
It's always the same thing: We take the earliest_possible and either
return 0 if the reference instant is larger, or the difference between
them. Fewer bugs, more readability, less code (:
Fix panic in wait_time_from - Fixes #22
NonConformance.wait_time_from as implemented by both the GCRA and the
leaky bucket would panic when given arguments that were later than the
time until which it was meant to wait; instead, return 0.