Change Gen members from def to val

[ashawley]

Code cherry-picked from #575

[ashawley]

This change probably won't show an improvement in a microbenchmark. The improvement should improve the runtime of a test suite in the large.

> 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   3399.136 ±  1451.612  us/op
GenBench.asciiPrintableStr        100          100  avgt    3   2029.820 ±    12.442  us/op
GenBench.const_                   100          100  avgt    3      4.210 ±     1.429  us/op
GenBench.double_                  100          100  avgt    3     12.538 ±     2.292  us/op
GenBench.dynamicFrequency         100          100  avgt    3   1993.503 ±   265.627  us/op
GenBench.eitherIntInt             100          100  avgt    3     55.811 ±     5.944  us/op
GenBench.identifier               100          100  avgt    3   4362.690 ±   557.519  us/op
GenBench.int_                     100          100  avgt    3     16.116 ±     0.816  us/op
GenBench.listOfInt                100          100  avgt    3   1924.777 ±   341.015  us/op
GenBench.mapOfIntInt              100          100  avgt    3   4339.788 ±   259.069  us/op
GenBench.oneOf                    100          100  avgt    3     27.448 ±    63.418  us/op
GenBench.optionInt                100          100  avgt    3     70.301 ±    15.245  us/op
GenBench.sequence                 100          100  avgt    3    289.889 ±    18.550  us/op
GenBench.staticFrequency          100          100  avgt    3   2132.837 ±  2517.276  us/op
GenBench.testFilter               100          100  avgt    3  26235.359 ± 71051.721  us/op
GenBench.zipIntInt                100          100  avgt    3     40.100 ±    57.710  us/op

[ashawley]

The build here still takes about 59 min on Travis, so probably no real change.