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/thread.h

@@ -56,13 +56,15 @@ class Thread {
     size_t                pvIdx, pvLast;
     std::atomic<uint64_t> nodes, tbHits, bestMoveChanges;
     int                   selDepth, nmpMinPly;
-    Value                 bestValue, optimism[COLOR_NB];
+    Value                 bestValue;
+
+    int optimism[COLOR_NB];
 
     Position              rootPos;
     StateInfo             rootState;
     Search::RootMoves     rootMoves;
     Depth                 rootDepth, completedDepth;
-    Value                 rootDelta;
+    int                   rootDelta;
     Value                 rootSimpleEval;
     CounterMoveHistory    counterMoves;
     ButterflyHistory      mainHistory;