[CM] Freeing up s7 instances & memory

[bil at ccrma.Stanford.EDU]

You're right that free(sc) does not free much.  Woody Douglass
sent me code to free most of s7's memory -- the main thing
is the heap -- but I haven't tackled merging it into my
version of s7 yet.  I think s7 instances can be reused --
just keep all your stuff in a sublet of rootlet, then
set it to #f, but it would be better to actually clean up.