`AsciiString::from_ascii` is unsound

[moulins]

An "evil" type can implement AsRef<[u8]> and Into<Vec<u8>> in inconsistent ways, which allows constructing a non-ASCII AsciiString:

use ascii::AsciiString;

struct EvilString(String);

impl AsRef<[u8]> for EvilString {
  fn as_ref(&self) -> &[u8] {
    ""
  }
}

impl From<EvilString> for Vec<u8> {
  fn from(s: EvilString) -> Self {
    s.0.into_bytes()
  }
}

fn unsound(s: String) -> AsciiString {
  // `s.as_ref()` is always empty and so valid ASCII, so this never fails, even if `s` contains non-ASCII characters.
  AsciiString::from_ascii(EvilString(s)).unwrap()
}