Clean up of Gen

[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   3343.834 ±   848.705  us/op
GenBench.asciiPrintableStr        100          100  avgt    3   2081.674 ±   234.164  us/op
GenBench.const_                   100          100  avgt    3      4.280 ±     0.485  us/op
GenBench.double_                  100          100  avgt    3     12.983 ±     1.300  us/op
GenBench.dynamicFrequency         100          100  avgt    3   2124.524 ±   263.651  us/op
GenBench.eitherIntInt             100          100  avgt    3     56.014 ±     1.844  us/op
GenBench.identifier               100          100  avgt    3   4641.965 ±   286.349  us/op
GenBench.int_                     100          100  avgt    3     16.878 ±     0.643  us/op
GenBench.listOfInt                100          100  avgt    3   1960.821 ±   227.542  us/op
GenBench.mapOfIntInt              100          100  avgt    3   4442.675 ±   144.927  us/op
GenBench.oneOf                    100          100  avgt    3     24.335 ±     3.146  us/op
GenBench.optionInt                100          100  avgt    3     67.003 ±     7.527  us/op
GenBench.sequence                 100          100  avgt    3    318.617 ±    81.490  us/op
GenBench.staticFrequency          100          100  avgt    3   2498.155 ±  6934.889  us/op
GenBench.testFilter               100          100  avgt    3  25201.300 ± 10382.093  us/op
GenBench.zipIntInt                100          100  avgt    3     49.424 ±    25.465  us/op

[ashawley]

Summary:

• Adding explicit type signatures
• Whitespace/line formatting changes
• Use Integer$.maxinstead of Integer.max