Graph Merging and Guards

[andrewCodeDev]

This is a way to make sub-graphs even more powerful. The subgraph macro by default creates a new scoped graph:

graph::@subgraph(...) {
    // on a separate graph
}

This is handy if you need to construct a leaf node for the upper level graph but it requires pre-computation. It has an even broader ability to destroy a graph if a failing operation could occur. This acts like scoped garbage collection.