[CM] Make a debugger in s7?

[bil at ccrma.Stanford.EDU]

In s7-scm.html, there's an example that uses repl.scm
and drop-into-repl.  repl.scm is using a terminal in
raw mode (or whatever the name is) and vt-100 codes.
There's another commented-out example in the html file
around line 883.  I think nrepl would also work here,
and would give mouse support.  debug.scm has a break
function, but it has been a long time since I used it.
(It's also described in s7-scm.html).