On the 'xml-writer' branch: Implement the xml writer api.

* subversion/include/svn_xml.h
  (svn_xml_writer_t,
  (svn_xml_writer_create,
   svn_xml_writer_close,
   svn_xml_writer_flush,
   svn_xml_write_open_tag,
   svn_xml_write_open_tag_v,
   svn_xml_write_open_tag_hash,
   svn_xml_write_cdata_cstring,
   svn_xml_write_cdata,
   svn_xml_write_close_tag,
   svn_xml_write_header): Declare symbols.

* subversion/libsvn_subr/xml_writer.c
  (xml_stream_write,
   xml_ensure_bytes,
   xml_write_byte,
   xml_write_bytes,
   xml_write_cstring): Implement functions for writing data to the buffer.

  (svn_xml_writer_create,
   svn_xml_writer_close,
   svn_xml_writer_flush,
   svn_xml_write_open_tag,
   svn_xml_write_open_tag_v,
   svn_xml_write_open_tag_hash,
   svn_xml_write_cdata_cstring,
   svn_xml_write_cdata,
   svn_xml_write_close_tag,
   svn_xml_write_header): Implement symbols.

* subversion/tests/libsvn_subr/xml-test.c
  (test_xml_writer,
   test_xml_writer_always_flush): New tests.
  (test_funcs): Run those tests.

Any kind of feedback will be much appreciated!!


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/xml-writer@1925686 13f79535-47bb-0310-9956-ffa450edef68