Improve the class generation benchmark by using the JMH framework.

Results:
Benchmark                             Mode  Cnt       Score      Error  Units
GeneratorBenchmark.asm4_0            thrpt   20  753119,277 ± 1890,328  ops/s
GeneratorBenchmark.asm5_0            thrpt   20  729869,073 ± 2328,108  ops/s
GeneratorBenchmark.asm6_0            thrpt   20  728330,694 ± 2244,506  ops/s
GeneratorBenchmark.asm6_1            thrpt   20  847886,985 ± 2767,943  ops/s
GeneratorBenchmark.aspectJBcel       thrpt   20  274363,976 ± 1171,862  ops/s
GeneratorBenchmark.bcel              thrpt   20  196715,573 ±  797,970  ops/s
GeneratorBenchmark.cojen             thrpt   20  191329,604 ±  898,662  ops/s
GeneratorBenchmark.csgBytecode       thrpt   20  325157,840 ± 1065,897  ops/s
GeneratorBenchmark.gnuByteCode       thrpt   20  245634,219 ± 1061,192  ops/s
GeneratorBenchmark.jclassLib         thrpt   20  280383,253 ±  973,727  ops/s
GeneratorBenchmark.jiapi             thrpt   20   75012,298 ±  325,473  ops/s
GeneratorBenchmark.mozillaClassFile  thrpt   20  303011,386 ± 1629,259  ops/s