Reduce std_unicode’s public API
* Only keep one copy of the `UTF8_CHAR_WIDTH` table instead of one of each of libcore and libstd_unicode.
* Move the `utf8_char_width` function to `core::str` under the `str_internals` unstable feature.
* Remove `std_unicode::str::is_utf16`. It was only accessible through the `#[unstable]` crate std_unicode. It has never been used in the compiler or standard library since 47e7a05 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).all(|r| r.is_ok())
}
```
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