[CM] fun with continuations

[Bill Schottstaedt]

You can interrupt with-sound now, and continue from the point
of the interrupt, look at local variables, etc; see ws.scm.
I also added a snd-break function to drop into the debugger
by hand, and snd-debug is much smarter.  All of these use
the normal Snd listener, so being in the debugger only
means you have some special stacks and continuations lying
around in case you want to refer to them.  This, with the
variable-display stuff added last week, should (I hope!)
make it much less painful to debug instruments and notelists.
I'm kinda jazzed by this -- Scheme's continuations seem like
black magic.

I'm interested in what else people want -- I could make an MCL-like
dialog with the stack/local variable info, etc.