RClass->m_tbl with serial. This prevents double marking method
  tables, since many classes/modules can share the same method table.
  This improves minor mark time in a large application by 30%.
* internal.h (struct method_table_wrapper): Define new
  wrapper struct with additional serial.
* internal.h (RCLASS_M_TBL_INIT): New macro for initializing method
  table wrapper and st_table.
* method.h (void rb_sweep_method_entry): Rename rb_free_m_table to
  rb_free_m_tbl for consistentcy
* .gdbinit (define rb_method_entry): Update rb_method_entry gdb helper
  for new method table structure.
* class.c: Use RCLASS_M_TBL_WRAPPER and
  RCLASS_M_TBL_INIT macros.
* class.c (rb_include_class_new): Share WRAPPER between module and
  iclass, so serial can prevent double marking.
* eval.c (rb_prepend_module): ditto.
* eval.c (rb_using_refinement): ditto.
* gc.c: Mark and free new wrapper struct.
* gc.c (obj_memsize_of): Count size of additional wrapper struct.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43973 b2dd03c8-39d4-4d8f-98ff-823fe69b080e