Implement test to clarify and fix iterator and subset relationships

Description: On reviewing the source code, it appears that the iterator
implementation for `ScriptExtension` only ever yields a single element. I
did not see this documented, and assuming this is a bug, I have written
a test case to clarify the expected subset, union, and iterator
behavior. If maintainers believe this behavior as shown in the test is
correct, I can  go ahead and implement the fixes in the rest of the
library.

Author: dcsommer

src/lib.rs

@@ -499,6 +499,49 @@ mod tests {
         assert!(scr.is_err());
     }
 
+    #[test]
+    fn test_subsets_and_iter() {
+        let cases: &[(ScriptExtension, &[Script])] = &[
+            (ScriptExtension::new_inherited(), &[Script::Inherited]),
+            (
+                ScriptExtension::new_common(),
+                &[Script::Inherited, Script::Common],
+            ),
+            (
+                ScriptExtension::new_inherited().union(script_extensions::LATIN),
+                &[
+                    Script::Inherited,
+                    Script::Common,
+                    Script::Inherited,
+                    Script::Latin,
+                ],
+            ),
+            (
+                ScriptExtension::new_inherited()
+                    .union(script_extensions::LATIN)
+                    .union(script_extensions::CYRILLIC),
+                &[
+                    Script::Inherited,
+                    Script::Common,
+                    Script::Inherited,
+                    Script::Latin,
+                    Script::Cyrillic,
+                ],
+            ),
+        ];
+        for &(full_extension, component_scripts) in cases {
+            for &script in component_scripts.iter() {
+                assert!(full_extension.contains_script(script));
+                let script_ext = script.into();
+                assert_eq!(full_extension.intersection(script_ext), script_ext);
+                assert_eq!(full_extension.union(script_ext), full_extension);
+            }
+            let scripts = component_scripts.iter().cloned().collect::<Vec<_>>();
+            let scripts_iterated = full_extension.iter().collect::<Vec<_>>();
+            assert_eq!(scripts, scripts_iterated);
+        }
+    }
+
     #[cfg(feature = "bench")]
     #[bench]
     fn bench_script_intersection(b: &mut Bencher) {