Better DAG test and topological ordering implementation

[Alpar Juttner]

Currently all these functions use Dfs with a custom visitor. I have a feeling that a direct algorithm might be faster and/or more memory efficient. The algorithm I propose is the following:

• Compute the in-degree of all nodes and store them in a NodeMap deg.
• Put all node n with deg[n]==0 into a queue (or a stack) q.
• While q is not empty:
  • Get a node n from g. This is the next node in the topological order.
  • For each out-arc a of n:
    • Decrease deg[target(a)] by 1
    • If deg[target(a)]==0 then put target(a) into q