Improve source/stream terminology consistency

We inherited the term "source" from vorbisfile's "datasource", but
 were using it interchangeably with stream.
At least one user did not even realize the that the _source
 parameter passed to op_open_callbacks() was the same as the
 _stream parameter taken by those callbacks, which is reasonable
 because we never said so.
Consistently use "stream" instead of "source" in both the
 documentation and the code.