Change Memory type-tags to enums, so they can't be instantiated (empt…

…y types)

gdnative-core/src/object/memory.rs

@@ -1,14 +1,13 @@
 //! Marker types to express the memory management method of Godot types.
+// Note: they are enums to prevent instantiation (empty types)
 
 use crate::object::bounds::MemorySpec;
 
 /// Marker that indicates that a type is manually managed.
-#[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash, Debug, Default)]
-pub struct ManuallyManaged;
+pub enum ManuallyManaged {}
 
 /// Marker that indicates that a type is reference-counted.
-#[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash, Debug, Default)]
-pub struct RefCounted;
+pub enum RefCounted {}
 
 /// Trait to parameterize over the memory management markers [`ManuallyManaged`] and [`RefCounted`].
 ///