* gc.c: tuning parameters.
* gc.c (GC_MALLOC_LIMIT): change default value to 16MB.
* gc.c (GC_MALLOC_LIMIT_GROWTH_FACTOR): change default value to 2.0.
* gc.c (gc_before_sweep): change decrease ratio of `malloc_limit'
  from 1/4 to 1/10.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43425 b2dd03c8-39d4-4d8f-98ff-823fe69b080e