* hash.c (rb_hash_keys): use st_keys() for performance improvement
if st_data_t and VALUE are compatible.
* st.h: define macro ST_DATA_COMPATIBLE_P() to predicate whether
st_data_t and passed type are compatible.
* configure.in: check existence of builtin function to use in
ST_DATA_COMPATIBLE_P().
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43885 b2dd03c8-39d4-4d8f-98ff-823fe69b080e