Fix lev_distance in libcollections/str.rs

There is an issue with lev_distance, where
```
fn main() {
    println!("{}", "\x80".lev_distance("\x80"))
}
```
prints `2`.

This is due to using the byte length instead of the char length.
dcol is too long, should be just .char_len() + 1. The difference is often small though.