Made MixinMemoryView constructor private and only callable by its implementation.

source/mijin/container/memory_view.hpp

@@ -41,6 +41,9 @@ concept RWMemoryViewable = MemoryViewable<T> && requires(T& object)
 template<typename TConcrete>
 class MixinMemoryView
 {
+private:
+    MixinMemoryView() = default;
+
 public:
     static constexpr bool WRITABLE = requires(TConcrete& object) { { object.data() } -> std::convertible_to<void*>; };
 
@@ -82,6 +85,8 @@ private:
     {
         return static_cast<const TConcrete*>(this)->byteSize();
     }
+
+    friend TConcrete;
 };
 
 class MemoryView : public MixinMemoryView<MemoryView>