Add hexadecimal formatting of integers with fmt::Debug

Add hexadecimal formatting of integers with fmt::Debug

This can be used for integers within a larger types which implements Debug
(possibly through derive) but not fmt::UpperHex or fmt::LowerHex.

```rust
assert!(format!("{:02x?}", b"Foo\0") == "[46, 6f, 6f, 00]");
assert!(format!("{:02X?}", b"Foo\0") == "[46, 6F, 6F, 00]");
```

RFC: https://github.com/rust-lang/rfcs/pull/2226