* st.c (st_keys): define st_keys() for performance improvement of
  Hash#keys and Array#uniq.

* st.h: ditto.

* hash.c (rb_hash_keys): use st_keys().

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43238 b2dd03c8-39d4-4d8f-98ff-823fe69b080e