* array.c (rb_ary_uniq): use rb_hash_keys().

* internal.h: define rb_hash_keys() as internal API.

* hash.c (rb_hash_keys): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43194 b2dd03c8-39d4-4d8f-98ff-823fe69b080e