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.
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