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/tt.h

@@ -53,7 +53,7 @@ struct TTEntry {
     uint16_t key16;
     uint8_t  depth8;
     uint8_t  genBound8;
-    uint16_t move16;
+    Move     move16;
     int16_t  value16;
     int16_t  eval16;
 };