Make it more convenient to switch on type of AccumulatorStatement

At the moment there is no easy way to switch on the type of an AccumulationStatement. This is because the information about the true/runtime "type" of the accumulator is hidden inside a factory function.

Before creating an SBE stage builder for accumulators we should make it convenient to do a visitor-like pattern on accumulators. We'll need to talk to QO to figure out the best way to do this.