stream.c: Added 2 more debugging utils and added pos to stream string

 * Added ast_stream_to_stra and ast_stream_topology_to_stra() macros
   which are shortcuts for
      ast_str_tmp(256, ast_stream_to_str(stream, &STR_TMP))

 * Added the stream position to the string representation of the
   stream.

 * Fixed some formatting in ast_stream_to_str().

Change-Id: Idaf4cb0affa46d4dce58a73a111f35435331cc4b