Multichild stages

Concept structure described as so:

Instead of linear stage execution, where

a -> b -> c -> d ->,

we want to insert support for multipath stage execution, where you can have

a -> b -> c -> d
b -> (desync) x -> y -> z

or a -> b(x -> y -> z) -> c -> d

or even a (choice)-> b
-> c
-> d

Obviously any implementation of this would be a huge breakthrough for our sister project Komponent