Implemented visit functions for the visit_thing_fn interface. The gc

check/mark functions are restored to how they were before the
GC_PASS_COUNT_MEMORY stuff.

Rev: src/array.c:1.206
Rev: src/array.h:1.74
Rev: src/constants.c:1.59
Rev: src/constants.h:1.36
Rev: src/mapping.c:1.200
Rev: src/mapping.h:1.67
Rev: src/multiset.c:1.110
Rev: src/multiset.h:1.43
Rev: src/object.c:1.290
Rev: src/object.h:1.95
Rev: src/pike_types.c:1.331
Rev: src/pike_types.h:1.114
Rev: src/program.c:1.691
Rev: src/program.h:1.242
Rev: src/stralloc.c:1.218
Rev: src/stralloc.h:1.103