Added more tracking in the gc to help track down bugs when the gc follows munged pointers in the mark pass. Activated with GC_MARK_DEBUG. Rev: src/array.c:1.135 Rev: src/gc.c:1.196 Rev: src/gc.h:1.95 Rev: src/mapping.c:1.160 Rev: src/object.c:1.221 Rev: src/program.c:1.478 Rev: src/svalue.c:1.156