Formalize the plan trace interface

[pipex]

Events happening during the planning process (next iteration, try
instruction, condition failed, etc.) are now defined as specific types
with accompanying data. These events can be organized as a state
machine, which means trace functions can use the state machine to
identify where in the process an event happens, and use that
information, for instance, to create visualizations of the decision
tree.

Change-type: minor

[pipex]

I self-certify!