Add an abstract MemberVisitor class to be able to reduce code duplication in ClassReader. Not sure if the class and/or its methods must be public.

Results of 3 benchmark runs with this change:

Benchmark                          Mode  Cnt    Score   Error  Units
AdapterBenchmark.read_asm7_1      thrpt   30  623,892 ± 1,506  ops/s
AdapterBenchmark.read_asmCurrent  thrpt   30  621,629 ± 1,835  ops/s

Benchmark                          Mode  Cnt    Score   Error  Units
AdapterBenchmark.read_asm7_1      thrpt   30  624,589 ± 1,260  ops/s
AdapterBenchmark.read_asmCurrent  thrpt   30  622,971 ± 2,541  ops/s

Benchmark                          Mode  Cnt    Score   Error  Units
AdapterBenchmark.read_asm7_1      thrpt   30  623,515 ± 1,680  ops/s
AdapterBenchmark.read_asmCurrent  thrpt   30  622,151 ± 1,619  ops/s

and without it:

Benchmark                          Mode  Cnt    Score   Error  Units
AdapterBenchmark.read_asm7_1      thrpt   30  624,141 ± 1,511  ops/s
AdapterBenchmark.read_asmCurrent  thrpt   30  625,117 ± 1,653  ops/s

Benchmark                          Mode  Cnt    Score   Error  Units
AdapterBenchmark.read_asm7_1      thrpt   30  607,326 ± 1,285  ops/s
AdapterBenchmark.read_asmCurrent  thrpt   30  621,902 ± 1,259  ops/s

Benchmark                          Mode  Cnt    Score   Error  Units
AdapterBenchmark.read_asm7_1      thrpt   30  623,548 ± 1,490  ops/s
AdapterBenchmark.read_asmCurrent  thrpt   30  621,814 ± 1,579  ops/s