Add support for variadic functions in container flow

[sauyon]

/cc @owen-mc

[smowton]

Like the concept, but it doesn't compile: Expected a predicate with result but found 'ParameterNode.isParameterOf(Callable c, int i)', which is a predicate without result

[smowton]

Thinking about this further, I don't think the dataflow lib will accept us defining a store step that also transits a function boundary. Instead we should store into a synthetic node representing the array, then pass the array to the callee.

Java takes this approach, calling the synthetic node ImplicitVarargsArray.