[CM] Freeing up s7 instances & memory

[bil at ccrma.Stanford.EDU]

I think you misunderstood my suggestion.  It has nothing to do
with mutexes or threads.  Take repl.scm for example.  It has a
top-level-let, initially (inlet), or its equivalent (sublet (rootlet)).
All evaluation and loading by default takes place in that let, not
the rootlet.  So:

/home/bil/cl/ repl
<1> (define x 32)
32
<2> (defined? 'x)
#t
<3> (defined? 'x (rootlet))
#f
<4> (set! (*repl* 'top-level-let) (inlet))
(inlet)
<5> (defined? 'x)
#f

So by resetting top-level-let we're back to the pristine start-up
state, just as if we had freed the initial repl and started a new one.
There are a few tricky points involving the built-in hooks, but you
can save their startup state, and re-establish them when you clear
out the top-level-let.  In the multi-s7 case, I'd have a pool of
s7's.  There are no mutexes in s7.c (except some that Kjetil uses
for debugging).