Evaluation Boundaries

[mitchmindtree]

In some cases it may be useful for some nodes to act as evaluation "boundaries". That is, traversal for push or pull evaluation cannot pass through the node. A couple examples that come to mind:

• Delay nodes require haulting evaluation so that it may be resumed again after a certain time period.
• StateSynchronisation (or better name) nodes - @freesig's idea for synchronising evaluation between push and pull evaluation sources executing at different rates.
• Gates where ongoing evaluation might depend on some value (this might be more related to #21).