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, that is why I think `<~` could work, but not `~>`, but it might still be dangerously close to the `~>`.
however, it might not be that common to do the attach thing, so it might be most readable to have it only as explicit `builder.withSource` ?