make eof part of stream interface

[ssfrr]

Currently we signal the end of the stream by returning less data than the user requested. (it will block the task until all the data is available or we hit EOF). Julia IO streams support the eof method, which I think simplifies the logic working with streams somewhat.

current approach:

buf = SampleBuf(Float32, CHUNK_FRAMES, 2)

while true
    n = read!(stream, buf, CHUNK_FRAMES)
    # process `n` frames from buf
    n == CHUNK_FRAMES || break
end

vs.

buf = SampleBuf(Float32, CHUNK_FRAMES, 2)

while !eof(stream)
    n = read!(stream, buf, CHUNK_FRAMES)
    # process `n` frames from buf
end

It would be pretty easy to add eof, the question is whether we modify the stream semantics to allow a short read when we're not at the end of the stream, and only rely on eof. My current feeling is that's a cleaner design, I'm mostly hesitant about the churn.