Refactor low-level UTF-16 decoding.

* Rename `utf16_items` to `decode_utf16`. "Items" is meaningless.
* Move it to `rustc_unicode::char`, exposed in `std::char`.
* Generalize it to any `u16` iterable, not just `&[u16]`.
* Make it yield `Result` instead of a custom `Utf16Item` enum that was isomorphic to `Result`. This enable using the `FromIterator for Result` impl.
* Add a `REPLACEMENT_CHARACTER` constant.
* Document how `result.unwrap_or(REPLACEMENT_CHARACTER)` replaces `Utf16Item::to_char_lossy`.

src/libstd/lib.rs

@@ -242,6 +242,7 @@
 #![feature(unicode)]
 #![feature(unique)]
 #![feature(unsafe_no_drop_flag, filling_drop)]
+#![feature(decode_utf16)]
 #![feature(vec_push_all)]
 #![feature(vec_resize)]
 #![feature(wrapping)]