Test the size of every struct defined here.

src/presence.rs

@@ -323,6 +323,13 @@ mod tests {
     use super::*;
     use compare_elements::NamespaceAwareCompare;
 
+    #[test]
+    fn test_size() {
+        assert_size!(Show, 1);
+        assert_size!(Type, 1);
+        assert_size!(Presence, 224);
+    }
+
     #[test]
     fn test_simple() {
         #[cfg(not(feature = "component"))]