`std_unicode::char::decode_utf16` should be in `core`

[lambda-fairy]

The decode_utf16 function currently lives in the std_unicode crate. But, as far as I can tell, this function doesn't depend on any Unicode tables and has no reason to be in this crate.

I suggest moving this function (and its associated iterator) into the core crate. This will allow for decoding UTF-16 text under #![no_std] in stable Rust.

[SimonSapin]

I agree that there is no reason for this not to be in libcore. Feel free to submit a PR and ping me for review.

More generally I wonder if we can rely of various platforms’ linkers to remove unused tables, and if so if there’s any reason to keep std_unicode from core at all. But that doesn’t need to block moving decode_utf16 specifically.

[SimonSapin]

#49698 fixes this.