Weak type of multiset indexing

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=7615

Reported by Martin Nilsson nilsson@fastmail.se

Even though the type of indexing a multiset is int(0..1), (operators.c:5711:  tFunc(tMultiset tMix, tInt01) ), the actual type is int.

> multiset m = (< 3,7,9 >);
> typeof(m[7]);
(1) Result: int

See also SSL.Context `encrypt_then_mac()