Replace logging and debugging infrastructure with GHC Events

[rrnewton]

trafficstars

This would be better than printf messages.

[simonmar]

Absolutely. We have a plan to make the binary event format extensible and to enable user-generated events to use the extensibility mechanism (rather than being restricted to just strings as they are now). So this should wait until the infrastructure changes are in place.

Another idea I had was to emit logging information that can be reconstructed into a graph that you can feed into dot, and get a visualisation of your graph.