Use type aliases instead of enums for Value types

The primary rationale behind this lies in the fact that enums were not
originally designed to be employed in the manner we currently utilize them.

The Value enum was used like a type alias throughout the code and was often
misused. Furthermore, changing the underlying size of the enum to int16_t broke
everything, mostly because of the operator overloads for the Value enum, were
causing data to be truncated. Since Value is now a type alias, the operator
overloads are no longer required.

Passed Non-Regression STC:
https://tests.stockfishchess.org/tests/view/6593b8bb79aa8af82b95b401
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 235296 W: 59919 L: 59917 D: 115460
Ptnml(0-2): 743, 27085, 62054, 26959, 807

closes https://github.com/official-stockfish/Stockfish/pull/4960

No functional change

src/movepick.h

@@ -171,7 +171,7 @@ class MovePicker {
                const CapturePieceToHistory*,
                const PieceToHistory**,
                const PawnHistory*);
-    MovePicker(const Position&, Move, Value, const CapturePieceToHistory*);
+    MovePicker(const Position&, Move, int, const CapturePieceToHistory*);
     Move next_move(bool skipQuiets = false);
 
    private:
@@ -190,7 +190,7 @@ class MovePicker {
     Move                         ttMove;
     ExtMove                      refutations[3], *cur, *endMoves, *endBadCaptures;
     int                          stage;
-    Value                        threshold;
+    int                          threshold;
     Depth                        depth;
     ExtMove                      moves[MAX_MOVES];
 };