[CM] s7_load_c_string

[bil at ccrma.Stanford.EDU]

> short of turning off the GC while i'm loading my C string, how can i
> debug this?

GC bugs can be hard to track down, but I always start by
running with S7_DEBUGGING=1.  This adds tons of checks
(s7 will run 5 to 10 times slower), and save GC info
about every cell (see the end of the s7_cell declaration).
That might provide enough info to find/fix the problem.