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

*On 2022-05-28 @recmo wrote in [`1ec2e50`](https://github.com/recmo/uint/commit/1ec2e50003548890260e8c06886f44fd826bc07b) “Recognize new tags”:*

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

```rust
/// # 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 {
if radix > 64 {
return Err(ParseError::InvalidRadix(radix));
}
let mut err = None;

```
*From [`src/string.rs:118`](https://github.com/recmo/uint/blob/1ec2e50003548890260e8c06886f44fd826bc07b/src/string.rs#L118)*