Runtime implementation of multisets has a cardinality limit

[fabiomadge]

Split off from #3988.

Dafny version

4.1.0

Code to produce this issue

method Main() {
    var x: multiset<int> := multiset{};
    x := x[1 := 18446744073709551616];
    print |x|;
}

What happened?

Crash

What type of operating system are you experiencing the problem on?

Mac

[stefan-aws]

Can confirm the following error on my Mac with 4.1.0:

Error: Execution resulted in exception: Exception has been thrown by the target of an invocation.
System.OverflowException: Value was either too large or too small for an Int32.
   at System.Numerics.BigInteger.op_Explicit(BigInteger value)
   at Dafny.MultiSet`1.get_Count()
   at _module.__default._Main(ISequence`1 __noArgsParameter)
   at __CallToMain.<>c__DisplayClass0_0.<Main>b__0()
   at Dafny.Helpers.WithHaltHandling(Action action)
   at __CallToMain.Main(String[] args)

[fabiomadge]

What about the fix?