Add debug function.
Benchmark: Extend output about AI's performance.
Improve AI for immovable ships.

There are now 2 phases.
In the 1st one, the AI tries to hit as many ships as possible to immobilize
but not sink them.
In the 2nd phase, the AI sinks the ships with the usual strategy.
Merge branch 'master' of https://github.com/zrho/afp

Conflicts:
	battleships/Logic/Benchmark.hs