Fixes [bug 3693].
Note that this is a (minor) API change (struct processing * ==> struct mapping *).
Added a few const's.

Rev: src/array.c:1.160
Rev: src/array.h:1.61
Rev: src/mapping.c:1.181
Rev: src/mapping.h:1.56
Rev: src/multiset.c:1.86
Rev: src/multiset.h:1.35
Rev: src/svalue.c:1.194
Rev: src/svalue.h:1.129