parallel instruments in CLM

[To: cmdist]

I've added a new kind of instrument to CLM: parallel, or
"sample-synchronous" to speak jargon.  Each call on such
an instrument spawns a sort of C process, all such processes
running together on the same sample.  See defpinstrument
in the documentation. These instruments are completely
compatible with normal (overlayed) instruments, and appear
to run just as fast.  From the user's point of view the
only difference is defpinstrument for definstrument and
the calls in the note list have to be ordered by begin
time.  The point of this was to make it possible for
instruments to communicate at run-time, but it may also
make it easy to add real-time controls.