* gc.c, parse.y: support generational Symbol relatetd marking.
  Each symbols has String objects respectively to represent
  Symbols.
  These objects are marked only when:
* full marking
* new symbols are added
  This hack reduce symbols (related strings) marking time.
  For example, on my Linux environment, the following code
  "20_000_000.times{''}"
  with 40k symbols (similar symbol number on Rails 3.2.14 app,
  @jugyo tells me) boosts, from 7.3sec to 4.2sec.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43255 b2dd03c8-39d4-4d8f-98ff-823fe69b080e