* gc.c: simplify threshold of GC caused by malloc_increase.
  Now, malloc_limit is increased/decreased by mysterious logic.
  This fix simplify malloc_limit increase/decrease logic such as:
  if (malloc_increase > malloc_limit) /* so many malloc */
  malloc_limit += malloc_limit * (GC_MALLOC_LIMIT_FACTOR-1);
  else
  malloc_limit -= malloc_limit * (GC_MALLOC_LIMIT_FACTOR-1)/4;
  Default value of GC_MALLOC_LIMIT_FACTOR is 1.8.
  malloc_limit is bounded by GC_MALLOC_LIMIT_MAX (256MB by default).
  This logic runs at before_sweeep(). So there are no effect from
  caused by lazy sweep. And we can remove malloc_increase2.
* gc.c (HEAP_MIN_SLOTS, FREE_MIN, HEAP_GROWTH_FACTOR): rename to
  GC_HEAP_MIN_SLOTS, GC_FREE_MIN, GC_HEAP_GROWTH_FACTOR respectively.
  Check them by `#ifndef' so you can specify these values outside gc.c.
* gc.c (ruby_gc_params_t): add initial_malloc_limit_factor and
  initial_malloc_limit_max.
* gc.c (vm_malloc_prepare, vm_xrealloc): use vm_malloc_increase to
  add and check malloc_increase.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43067 b2dd03c8-39d4-4d8f-98ff-823fe69b080e