Better BaseLayerIterator

[robinhundt]

Currently, we have a BaseLayerIterator type which calculates partitions of a base circuit for each distinct use of a base (sub) circuit. However, I think we can do better by this by employing the visitor pattern. The benefit of this would be that we don't need to construct an intermediate collection which holds all the information for the current layer.

An open question is how to handle the interactive gates, as we need to iterate twice over these, once to construct the msg and once after exchanging the message.