obtain some outputs/or other node.

In order for scan to identify which of the inner outputs is the stopping
condition I use this dummy class to wrap my condition in it.

Look for conditions that the user might provide ( if scan is to terminate on
a condition ). This feature will be later used the conditional terminating
scan.

One thing that I've tried is to avoid importing cuda.

The main thing I was looking for was efficiency. The new algorithm
is not recursive, it considers a bunch of outputs at the same time,
and it uses toposort to traverse the graph, making sure each node is seen
only once.

safe_to_cpu was one of the reason that I was importing cuda. It was a
misunderstanding on my part, and the way is done now is somewhat more sane.

added)

I've removed the old way of writing tests, and added a more generic way of
generating those tests. The diff got all messed up, so I committed
everything at once.