* Rename `Utf16Items` to `Utf16Decoder`. "Items" is meaningless.
* Generalize it to any `u16` iterator, not just `[u16].iter()`
* Make it yield `Result` instead of a custom `Utf16Item` enum that was isomorphic to `Result`. This enable using the `FromIterator for Result` impl.
* Replace `Utf16Item::to_char_lossy` with a `Utf16Decoder::lossy` iterator adaptor.
This is a [breaking change], but only for users of the unstable `rustc_unicode` crate.
I’d like this functionality to be stabilized and re-exported in `std` eventually, as the "low-level equivalent" of `String::from_utf16` and `String::from_utf16_lossy` like #27784 is the low-level equivalent of #27714.
CC @aturon, @alexcrichton
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