Use apr functions when available for functions that we expected to be
implemented by apr some day.

* subversion/libsvn_subr/iter.c
  (svn__apr_hash_index_key,
   svn__apr_hash_index_klen,
   svn__apr_hash_index_val): Use apr equivalent when available.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1542542 13f79535-47bb-0310-9956-ffa450edef68