Specialize std::hash<T> for Cell, Vector2, Vector3
This was lost in the migration from Boost to the standard library as boost::hash<T>
(and thus boost::unordered_map<T,V>
) uses hash_value<T>
if available and std::hash<T>
(and thus std::unordered_map<T,V>
) does not.
Regression introduced in !184 (merged)