Add functions to check for decisive scores

Thanks to peregrineshahin and robbyrobbyrob for their suggestions.

closes https://github.com/official-stockfish/Stockfish/pull/5696

No functional change

src/score.cpp

@@ -29,7 +29,7 @@ namespace Stockfish {
 Score::Score(Value v, const Position& pos) {
     assert(-VALUE_INFINITE < v && v < VALUE_INFINITE);
 
-    if (std::abs(v) < VALUE_TB_WIN_IN_MAX_PLY)
+    if (!is_decisive(v))
     {
         score = InternalUnits{UCIEngine::to_cp(v, pos)};
     }