* st.c: add st_values() and st_values_check().

* include/ruby/st.h: add prototypes for above.

* hash.c (rb_hash_values): use st_values_check() for performance
 improvement if VALUE and st_data_t are compatible.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43895 b2dd03c8-39d4-4d8f-98ff-823fe69b080e