[CM] pre-parsing scheme code

[bil at ccrma.Stanford.EDU]

I added that example to s7.html, and added GC protection
for "body".  I didn't advertise s7_optimize in the past
because I was fiddling with it for a long time.  There
is also s7_float_optimize which is probably not
documented, and others like it in s7.c.  If s7_optimize
returns NULL, you can fall back on s7_eval (NULL just
means the optimizer gave up for some reason).