Prototype of FlowGraph

Early prototype for discussing the way forward.
Before we start with the DSL sugar I would like that we express the functionality in terms of something similar to the raw `addEdge` methods.
yes, EdgeBuilder is not the final name. I think we should have one FlowGraphBuilder, which possibly incorporate ideas from both styles of constructing the graph.

`b1.merge` factory is not really needed, so far, the case class can be used directly