Refactor Stats Array

* Limit use of `StatsEntry` wrapper to arithmetic types
* Generalize `Stats` to `MultiArray` by discarding the template parameter `D`
* Allow `MultiArray::fill` to take any type assignable to element type
* Remove now-unused operator overloads on `StatsEntry`

closes https://github.com/official-stockfish/Stockfish/pull/5750

No functional change

src/search.cpp

@@ -510,13 +510,13 @@ void Search::Worker::clear() {
 
     for (auto& to : continuationCorrectionHistory)
         for (auto& h : to)
-            h->fill(0);
+            h.fill(0);
 
     for (bool inCheck : {false, true})
         for (StatsType c : {NoCaptures, Captures})
             for (auto& to : continuationHistory[inCheck][c])
                 for (auto& h : to)
-                    h->fill(-427);
+                    h.fill(-427);
 
     for (size_t i = 1; i < reductions.size(); ++i)
         reductions[i] = int(19.43 * std::log(i));