load.c: loaded_features_index st_table

* load.c (rb_feature_p), vm_core.h (rb_vm_struct): turn
  loaded_features_index into st_table.  patches by tmm1 (Aman Gupta)
  in [ruby-core:53251] and [ruby-core:53274] [Bug #8048]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39874 b2dd03c8-39d4-4d8f-98ff-823fe69b080e