Use custom closure structs for the predicates so that the iterator's
clone can simply be derived. This should also reduce virtual call
overhead by not using function pointers.
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())
}
```
