Replace tree-based semantics by DAG based one

[javra]

Right now series of confluent branchings blow up the generated term exponentially. This path explosion issue is not really necessary if we replace the use of AndOrTree in the semantics by a suitable data structure of series-parallel graphs that treat confluence points correctly (exactly the Sierra statements containing branch_align). The control flow of process_state in Aegis.Analyzer has to be changed to account for that.