Auto merge of #110729 - ColinFinck:decode-utf16-fused-iterator, r=dtolnay

Implement FusedIterator for DecodeUtf16 when the inner iterator does

I have just implemented an iterator that wraps `DecodeUtf16` and wanted to implement `FusedIterator` for my iterator when I noticed that `DecodeUtf16` currently doesn't implement `FusedIterator` at all.
A quick look at the code of `DecodeUtf16` revealed that `DecodeUtf16::next` only returns `None` when its inner iterator returns `None`:
https://github.com/rust-lang/rust/blob/3462f79e94f466a56ddaccfcdd3a3d44dd1dda9f/library/core/src/char/decode.rs#L45

As a result, we can implement `FusedIterator` for `DecodeUtf16` when the inner iterator does.

I'm following the example of #96397 here and consider this change minor and non-controversial, which is why I haven't added an RFC. I have also added the required feature name (`"decode_utf16_fused_iterator"`), however without adding a chapter to the Rust Unstable book (same as #96397).

Author: bors