Shrinking time limit

When testing Onebip's rounding feature shrinking took several minutes without the user being aware of what was happening. This PR gives the user the ability to set a time limit for shrinking so that he can trade-off a more complex failing input for execution time.

The default limit is infinite.
Is PHP implementation of `time/0` wall clock drift tolerant or our intervals can be screwed by the wall clock? :smile_cat: Just kidding obviously.