Daring another no-no: provide our own, optimized implementation
of a string to unsigned int conversion.  Use it to simplify & speed up
svn_revnum_parse.

Background: In my 'svn log -g' tests, I found that >50% of the runtime
is spent parsing mergeinfo and ~20% in strtol alone.

* subversion/include/private/svn_string_private.h
  (svn__strtoff): declare new private API

* subversion/libsvn_subr/string.c
  (svn__strtoff): implement it

* subversion/libsvn_subr/types.c
  (svn_revnum_parse): use it


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1483292 13f79535-47bb-0310-9956-ffa450edef68