[CM] Backtrace in s7

[Bill Schottstaedt]

Thanks very much for the praise!

> Is it possible to get this type of backtrace at all? 

Not without slightly rewriting the scheme code -- those are all
tail-calls, so there is nothing in the s7 stack that represents the
call sequence. Wrap everything in dynamic-winds or the equivalent
to get a C-like stacktrace.

> The feature is to give
> warnings for undefined variables and functions.

Ok -- I'll add a switch for this -- did Guile (Andy Wingo?) 
say why they chose to handle these with warnings?