Deterministically vary the number of seed slides in Test.check

[leviramsey]

(re-attempt of #841, after that was inadvertently turned into an empty PR in the process of moving to a feature branch in my fork)

A Prop.forAll of multiple parameters will use the initial seed to generate the first parameter and slide the seed once for each of the subsequent parameters. When combined with Test.check which slides the seed before evaluating the property, this means that for a property with m > 1 parameters, if the i th evaluation of the property had seeds (seed1 ... seedm), then the i+1 th evaluation will have seeds (seed2 ... seedm, seedm.slide). If the same generator is used for multiple parameters, then a generated value in one evaluation will be reused in a different position in a specific later evaluation.

This addresses this by varying the number of slides done in Test.check based on the number of times a given property has been evaluated. The number of slides remains deterministic, just less amenable to pattern recognition.

The seed sliding behavior between checks is configurable:

• If maxRNGSpins is 1, 0, or negative: 1 spin is performed
• Otherwise at most maxRNGSpins will be performed; the number of spins is deterministic and as the number of checks of a given property increases, the total number of spins performed between checks converges to N * (maxRNGSpins + 1)/2

[leviramsey]

Addresses #840 and scalatest/scalatest#2054

[SethTisue]

Just one more thing before we merge — for the history, does this make sense as 6 commits?