Change the order of the arguments of svn_stream_span() to be consistent
with the other generic stream functions.

* subversion/include/svn_io.h
  (svn_stream_span): Put the 'stream' argument first.

* subversion/tests/libsvn_subr/stream-test.c,
  subversion/libsvn_subr/stream.c,
  subversion/libsvn_subr/subst.c: : Update implementations and usage.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/patch-from-stream@1925680 13f79535-47bb-0310-9956-ffa450edef68