Clean up Arbitrary

[ashawley]

Code cherry-picked from #575

[ashawley]

No surprise there was no change in the benchmark:

> bench/jmh:run -wi 3 -i 3 -f1 -t1 org.scalacheck.bench.GenBench
Benchmark                   (genSize)  (seedCount)  Mode  Cnt      Score       Error  Units
GenBench.arbitraryString          100          100  avgt    3   3256.892 ±   162.667  us/op
GenBench.asciiPrintableStr        100          100  avgt    3   2075.132 ±   165.350  us/op
GenBench.const_                   100          100  avgt    3      4.319 ±     1.634  us/op
GenBench.double_                  100          100  avgt    3      7.265 ±     2.357  us/op
GenBench.dynamicFrequency         100          100  avgt    3   2098.918 ±   807.409  us/op
GenBench.eitherIntInt             100          100  avgt    3     55.262 ±     5.898  us/op
GenBench.identifier               100          100  avgt    3   4624.433 ±   358.232  us/op
GenBench.int_                     100          100  avgt    3     17.317 ±     2.135  us/op
GenBench.listOfInt                100          100  avgt    3   1921.500 ±   137.676  us/op
GenBench.mapOfIntInt              100          100  avgt    3   4494.749 ±  5888.692  us/op
GenBench.oneOf                    100          100  avgt    3     24.666 ±     2.471  us/op
GenBench.optionInt                100          100  avgt    3     74.330 ±    10.861  us/op
GenBench.sequence                 100          100  avgt    3    286.318 ±    12.909  us/op
GenBench.staticFrequency          100          100  avgt    3   2027.246 ±   455.212  us/op
GenBench.testFilter               100          100  avgt    3  23813.270 ± 22566.081  us/op
GenBench.zipIntInt                100          100  avgt    3     40.187 ±    81.477  us/op