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
The new formatting string syntax (`x?` and `X?`) is insta-stable in this PR because I don’t know how to change a built-in proc macro’s behavior based of a feature gate. I can look into adding that, but I also strongly suspect that keeping this feature unstable for a time period would not be useful as possibly no-one would use it during that time.
This PR does not add the new (public) `fmt::Formatter` proposed in the API because:
* There was some skepticism on response to this part of the RFC
* It is not possible to implement as-is without larger changes to `fmt`, because `Formatter` at the moment has no easy way to tell apart for example `Octal` from `Binary`: it only has a function pointer for the relevant `fmt()` method.
If some integer-like type outside of `std` want to implement this behavior, another RFC will likely need to propose a different public API for `Formatter`.
152217d29c