Prepitch: Character integer literals

[SDGGiesbrecht (Jeremy David Giesbrecht)]

@Nevin

If n is an integer written in hexadecimal, then "\u{n}" either uniquely determines a one-character string, or it is an error.

If it is not an error, then the resulting one-character string is uniquely determined by the integer n .

assert("\u{3B}" != "\u{37E}", "...oh...")