lib/ai: add the eight-queen problem as an example

Signed-off-by: Jean Privat <jean@pryen.org>
friendz: use ai::backtrack instead of the ad-hoc solver

Signed-off-by: Jean Privat <jean@pryen.org>
friendz: add solver_cmd for solve from command line

Signed-off-by: Jean Privat <jean@pryen.org>