Make explicit `std::hash` specialization for ResourceKey.

Allows users to use ResourceKey in `std::unordered_map` more
conveniently.

src/Magnum.h

@@ -419,4 +419,11 @@ class Timeline;
 
 }
 
+namespace std {
+    #ifndef DOXYGEN_GENERATING_OUTPUT
+    template<class> struct hash;
+    #endif
+    template<> struct hash<Magnum::ResourceKey>;
+}
+
 #endif

src/Resource.h

@@ -277,6 +277,17 @@ template<class T, class U> void Resource<T, U>::acquire() {
 
 }
 
+namespace std {
+    /** @brief `std::hash` specialization for @ref Magnum::ResourceKey */
+    template<> struct hash<Magnum::ResourceKey> {
+        #ifndef DOXYGEN_GENERATING_OUTPUT
+        std::size_t operator()(Magnum::ResourceKey key) const {
+            return *reinterpret_cast<const std::size_t*>(key.byteArray());
+        }
+        #endif
+    };
+}
+
 /* Make the definition complete */
 #include "ResourceManager.h"
 

src/ResourceManager.h

@@ -91,12 +91,6 @@ template<class> class AbstractResourceLoader;
 
 namespace Implementation {
 
-struct ResourceKeyHash {
-    std::size_t operator()(ResourceKey key) const {
-        return *reinterpret_cast<const std::size_t*>(key.byteArray());
-    }
-};
-
 template<class T> class ResourceManagerData {
     template<class, class> friend class Magnum::Resource;
     friend class AbstractResourceLoader<T>;
@@ -147,7 +141,7 @@ template<class T> class ResourceManagerData {
 
         void decrementReferenceCount(ResourceKey key);
 
-        std::unordered_map<ResourceKey, Data, ResourceKeyHash> _data;
+        std::unordered_map<ResourceKey, Data> _data;
         T* _fallback;
         AbstractResourceLoader<T>* _loader;
         std::size_t _lastChange;