It was only accessible through the `#[unstable]` crate std_unicode.
It has never been used in the compiler or standard library
since 47e7a05a28 added it in 2012
“for OS API interop”.
It can be replaced with a one-liner:
```rust
fn is_utf16(slice: &[u16]) -> bool {
std::char::decode_utf16(s.iter().cloned()).all(|r| r.is_ok())
}
```
