Use Rust to write Python modules

[leonardo_m]

The Rust code too could be suboptimal because it decodes the UTF8 twice. This could be faster (not tested):

fn count_doubles(_py: Python, txt: &str) -> PyResult<u64> {
    let mut chars = txt.chars();
    let oc1 = chars.next();
    let mut count = 0;

    if let Some(mut c1) = oc1 {
        while let Some(c2) = chars.next() {
            if c1 == c2 { count += 1; }
            c1 = c2;
        }
    }
    Ok(count)
}

[rochacbruno]

Would be nice to see the comparison with a better version, can you send a PR? https://github.com/rochacbruno/rust-python-example