Finish conversion to entity id's for units.
Be able to correctly print actions again.

The switch to entity id's required a GameState reference to get the
names of the units involved.
GameState should only allow const handles to Units.

We're not there yet.  Animations need to set the unit's facing.  If we
can store that elsewhere, we won't have a need for non-const handles to
Units outside of GameState.
Log the action that will improve score the most.
Start putting AI code in its own file.
Added AI function with lookahead (untested).