slight search optimisation in case of a single top-level move available
make more methods private to the Board class
Added human player to actor based match app mix
Renamed file to match object inside it
minor refactor of Negamax
No need to maintain mapping to moves in Negamax.iter
more idiomatic
rename Fitness.fitness -> Fitness.apply
Added test for reference implementation of Alphabeta with stubbed-out test states & moves

Copy-paste implementation of Alphabeta from reference impl
use a 4-ply alphabeta search rather than negamax