Fixed tricky gc problems by implementing a gc-tailored call stack that can

be rotated. That also made the whole thing quite a bit less hairy.

Rev: src/array.c:1.78
Rev: src/array.h:1.21
Rev: src/gc.c:1.107
Rev: src/gc.h:1.55
Rev: src/mapping.c:1.93
Rev: src/mapping.h:1.27
Rev: src/multiset.c:1.25
Rev: src/multiset.h:1.13
Rev: src/object.c:1.136
Rev: src/object.h:1.49
Rev: src/program.c:1.251
Rev: src/program.h:1.96
Rev: src/svalue.c:1.84
Rev: src/testsuite.in:1.314