Multiset implementation using rbtree. --with-new-multisets required to

enable it. There's still no glue to use most of the new features, except
that the iterator (and foreach) works without locking the multiset index,
and that multiset data blocks use copy-on-write (just like mappings).

Rev: src/acconfig.h:1.94
Rev: src/builtin.cmod:1.71
Rev: src/builtin_functions.c:1.416
Rev: src/configure.in:1.562
Rev: src/docode.c:1.137
Rev: src/encode.c:1.133
Rev: src/gc.c:1.178
Rev: src/interpret.c:1.253
Rev: src/iterators.cmod:1.15
Rev: src/main.c:1.138
Rev: src/multiset.c:1.40
Rev: src/multiset.h:1.20
Rev: src/opcodes.c:1.117
Rev: src/operators.c:1.143
Rev: src/pike_types.c:1.183