* gc.c (Init_GC): Add new GC::INTERNAL_CONSTANTS for information about
  GC heap/page/slot sizing.
* test/ruby/test_gc.rb (class TestGc): test for above.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43877 b2dd03c8-39d4-4d8f-98ff-823fe69b080e