Allow for unicode literals in character literals: \u and \U

Some sample text to test the expression:

    let trim (str:String) =
        str.Split( [| '\n' ; '\r' |], StringSplitOptions.RemoveEmptyEntries)
    '\u1234'
    let bep = '\U1A343214'
    let ``shouldnt catch`` = '\uwerf' <-- not legal