AK: Make `Vector::contains_slow` templated

This allows for calling this function with any argument type for which
the appropriate traits and operators have been implemented so it can be
compared to the Vector's item type

AK/Vector.h

@@ -203,7 +203,8 @@ public:
         return TypedTransfer<StorageType>::compare(data(), other.data(), size());
     }
 
-    bool contains_slow(VisibleType const& value) const
+    template<typename V>
+    bool contains_slow(V const& value) const
     {
         for (size_t i = 0; i < size(); ++i) {
             if (Traits<VisibleType>::equals(at(i), value))