string: Support proper unicode. Ignore zero-width spaces, joiners, etc.

[github-actions]

On 2022-05-28 @recmo wrote in 1ec2e50 “Recognize new tags”:

Support proper unicode. Ignore zero-width spaces, joiners, etc.
Recognize digits from other alphabets.

    /// # Errors
    ///
    /// * [`ParseError::InvalidDigit`] if the string contains a non-digit.
    /// * [`ParseError::InvalidRadix`] if the radix is larger than 64.
    /// * [`ParseError::BaseConvertError`] if [`Uint::from_base_be`] fails.
    // FEATURE: Support proper unicode. Ignore zero-width spaces, joiners, etc.
    // Recognize digits from other alphabets.
    pub fn from_str_radix(src: &str, radix: u64) -> Result<Self, ParseError> {
        if radix > 64 {
            return Err(ParseError::InvalidRadix(radix));
        }
        let mut err = None;

From src/string.rs:118