st.c: revert st_keys

* st.c: revert st_keys() at r43238.  VALUE cannot be in st.c.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43243 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-10-11

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43244 b2dd03c8-39d4-4d8f-98ff-823fe69b080e