Switch to self instead of Gen.this

[ashawley]

Code cherry-picked from #575

[ashawley]

This shouldn't change performance either, but just to be sure:

> 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   3509.829 ±  4449.755  us/op
GenBench.asciiPrintableStr        100          100  avgt    3   2075.803 ±   648.052  us/op
GenBench.const_                   100          100  avgt    3      4.289 ±     1.790  us/op
GenBench.double_                  100          100  avgt    3     12.758 ±     2.255  us/op
GenBench.dynamicFrequency         100          100  avgt    3   2054.676 ±   910.133  us/op
GenBench.eitherIntInt             100          100  avgt    3     59.577 ±   121.032  us/op
GenBench.identifier               100          100  avgt    3   4598.152 ±  1794.132  us/op
GenBench.int_                     100          100  avgt    3     16.283 ±     0.308  us/op
GenBench.listOfInt                100          100  avgt    3   1943.837 ±   604.833  us/op
GenBench.mapOfIntInt              100          100  avgt    3   4507.883 ±  6738.781  us/op
GenBench.oneOf                    100          100  avgt    3     25.058 ±    11.399  us/op
GenBench.optionInt                100          100  avgt    3     68.185 ±    17.478  us/op
GenBench.sequence                 100          100  avgt    3    304.392 ±   297.912  us/op
GenBench.staticFrequency          100          100  avgt    3   2395.711 ±  1302.816  us/op
GenBench.testFilter               100          100  avgt    3  25521.167 ± 32727.678  us/op
GenBench.zipIntInt                100          100  avgt    3     36.341 ±     6.253  us/op