csharp: Change policy on ptr(struct) owned calls

When transferring the ownership of a ptr(struct) from Unamanaged to
managed, we should marshal the reference-typed fields or they can point
to Managed memory that would make no sense to access from C (for
example, strings would return garbage).

The downside is that it can cause potential leaks if the receiver of the
memory do not free it. In the current state of the EFL api this can
happen in two places:

- Efl.Ui.List.SegArray::remove
return -> ptr(Efl.Ui.List.LayoutItem) @owned
- Efl.Ui.Focus.Manager::fetch
return -> ptr(Efl.Ui.Focus.Relations) @owned

The resources copied by both structs may leak when those functions are
overriden in C# and the values returned to the C code.

Also hide some internal stuff instead of exporting it and generate
implicit conversion operators for struct external/internal.

src/bindings/mono/eina_mono/eina_common.cs

@@ -110,11 +110,9 @@ public static class PrimitiveConversion
 
    public static IntPtr ManagedToPointerAlloc<T>(T man)
    {
-       GCHandle pinnedData = GCHandle.Alloc(man, GCHandleType.Pinned);
-       IntPtr ptr = pinnedData.AddrOfPinnedObject();
-       IntPtr nat = MemoryNative.AllocCopy(ptr, Marshal.SizeOf<T>());
-       pinnedData.Free();
-       return nat;
+       IntPtr ptr = Marshal.AllocHGlobal(Marshal.SizeOf<T>());
+       Marshal.StructureToPtr(man, ptr, false);
+       return ptr;
    }
 }