Slight speep up fetching the endgame table

Replace calls to count(key) + operator[key] with a single call to find(key).
Replace the std::map with std::unordered_map which provide O(1) access,
although the map has a really small number of objects.

Test with [0..4] failed yellow:

TC	10+0.1
SPRT	elo0: 0.00  alpha: 0.05  elo1: 4.00  beta: 0.05
LLR	-2.96 [-2.94,2.94] (rejected)
Elo	1.01 [-0.87,3.08] (95%)
LOS	85.3%
Games	71860 [w:22.3%, l:22.2%, d:55.5%]
http://tests.stockfishchess.org/tests/view/5d5432210ebc5925cf109d61

Closes #2269

No functional change

src/endgame.h

@@ -21,7 +21,7 @@
 #ifndef ENDGAME_H_INCLUDED
 #define ENDGAME_H_INCLUDED
 
-#include <map>
+#include <unordered_map>
 #include <memory>
 #include <string>
 #include <type_traits>
@@ -98,7 +98,7 @@ struct Endgame : public EndgameBase<T> {
 namespace Endgames {
 
   template<typename T> using Ptr = std::unique_ptr<EndgameBase<T>>;
-  template<typename T> using Map = std::map<Key, Ptr<T>>;
+  template<typename T> using Map = std::unordered_map<Key, Ptr<T>>;
 
   extern std::pair<Map<Value>, Map<ScaleFactor>> maps;
 
@@ -119,7 +119,8 @@ namespace Endgames {
 
   template<typename T>
   const EndgameBase<T>* probe(Key key) {
-    return map<T>().count(key) ? map<T>()[key].get() : nullptr;
+    auto it = map<T>().find(key);
+    return it != map<T>().end() ? it->second.get() : nullptr;
   }
 }