[CM] R7RS in S7

[bil at ccrma.Stanford.EDU]

I can't resist responding to your comment that s7 is
fast (which has me smiling from ear to ear): even though
I've made little effort to "game" the standard benchmarks,
if you use the ranking system the ecraven site uses,
https://ecraven.github.io/r7rs-benchmarks/, s7 comes
in 4th among all schemes, compiled or interpreted.
Chez, Gambit, and Cyclone are faster.  Maybe it's
tied with MIT-Scheme.