On the fsfs-format7 branch:  Introduce a new serialization utility
function that slightly speeds up storing various data in our caches.

* subversion/include/private/svn_temp_serializer.h
  (svn_temp_serializer__add_leaf): declare the new API function

* subversion/libsvn_subr/temp_serializer.c
  (svn_temp_serializer__add_leaf): implement it

* subversion/libsvn_fs_fs/temp_serializer.c
  (serialize_svn_string,
   serialize_checksum,
   serialize_txdelta_ops,
   svn_fs_fs__serialize_mergeinfo): simplify using the new function

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1442068 13f79535-47bb-0310-9956-ffa450edef68