Add Utf16Encoder. Generalize Utf16CodeUnits for any Iterator<char>.

This allows encoding to UTF-16 something that is not in UTF-8, e.g. a
`[char]` UTF-32 string.

This might help with https://github.com/servo/servo/issues/4023

src/libcoretest/str.rs

@@ -114,3 +114,10 @@ fn test_rev_split_char_iterator_no_trailing() {
     split.reverse();
     assert_eq!(split, vec!["", "Märy häd ä little lämb", "Little lämb"]);
 }
+
+#[test]
+fn test_utf16_code_units() {
+    use core::str::Utf16Encoder;
+    assert_eq!(Utf16Encoder::new(vec!['é', '\U0001F4A9'].into_iter()).collect::<Vec<u16>>(),
+               vec![0xE9, 0xD83D, 0xDCA9])
+}