On the fsfs-format7 branch:  Introduce uint <-> string conversion
functions that operate on base36 strings as they are used within
our node, copy and txn ids.

This is the first of a long list of commits that tries to break
down the 5k lines patch I wrote to replace the internal usage of
string IDs with numerical IDs.

* subversion/include/private/svn_string_private.h
  (svn__ui64tobase36,
   svn__base36toui64): declare new private API functions

* subversion/libsvn_subr/string.c
  (svn__ui64tobase36,
   svn__base36toui64): implement them

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1453590 13f79535-47bb-0310-9956-ffa450edef68