Really remove PolicyGenerator for now
(cherry picked from commit fbfcc09b9dfa1ba9a1d91bbff4d21b5889a132c5)
from_pipe: port to latest topology API
from_pipe: Different eval strategies for union

If a branch of a union has side-effects, the user may want to ensure their
ordering by using `Sequential, which will evaluate the left application to
completion before running the right one. `Parallel provides no guarantee about
evaluation order.
Revert "from_pipe: remove unused port in packet_out"

This reverts commit 8ba2bd01e03666774038198e86712426ce54f27f.

Conflicts:
	lib/NetKAT_Types.ml