the ruby heap. [Bug #9201] [ruby-core:58805]
* load.c (loaded_features_index_clear_i): Clean up off-heap array structure.
* vm.c (rb_vm_mark): Remove unnecessary mark_tbl for loaded_features_index.
  This improves minor GC time by 15% in a large application.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43974 b2dd03c8-39d4-4d8f-98ff-823fe69b080e