Add functionality to compose streamers

Hi!

I'm currently working on a library, which generates binaural sound and use MIXALOT as a framework.
So, I needed to be able to impose effects on arbitrary streamers, hence, added to MIXALOT functionality,
that allows that.

1. DUMMY-MIXER class - to be fed as MIXER parameter in STREAMER-MIX-INTO and STREAMER-WRITE-INTO,
when I call these methods from another streamer and not from the real mixer.
2. PLAYBACK-FINISHED condition, which contains number of samples actually written by STREAMER-MIX-INTO
and STREAMER-WRITE-INTO, together with convenience macro.

Hope this would be useful,
BRGDS,
Alex