Change the Move enum to a class

This changes the Move enum to a class, this way
all move related functions can be moved into the class
and be more self contained.

closes https://github.com/official-stockfish/Stockfish/pull/4958

No functional change

src/movepick.h

@@ -142,7 +142,7 @@ using CorrectionHistory =
 // MovePicker class is used to pick one pseudo-legal move at a time from the
 // current position. The most important method is next_move(), which returns a
 // new pseudo-legal move each time it is called, until there are no moves left,
-// when MOVE_NONE is returned. In order to improve the efficiency of the
+// when Move::none() is returned. In order to improve the efficiency of the
 // alpha-beta algorithm, MovePicker attempts to return the moves which are most
 // likely to get a cut-off first.
 class MovePicker {