load.c: reduce indexes arrays

* load.c (features_index_add_single, rb_feature_p): store single index
  as Fixnum to reduce the number of arrays for the indexes.  based on
  the patch by tmm1 (Aman Gupta) in [ruby-core:53216] [Bug #8048].

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39644 b2dd03c8-39d4-4d8f-98ff-823fe69b080e