feat: Add binary_elementwise_into_string_amortized

[condekind]

We already have apply_into_string_amortized and binary_elementwise. The contributed function is a specialized version that combines the logic of these two. Here's an example of how it can simplify some scenarios. This code is from a plugin that performs reverse geocoding:

#[polars_expr(output_type=String)]
fn reverse_geocode(inputs: &[Series]) -> PolarsResult<Series> {
    let lat = inputs[0].f64()?;
    let lon = inputs[1].f64()?;
    let geocoder = ReverseGeocoder::new();

    let (lhs, rhs) = align_chunks_binary(lat, lon);
    let chunks = lhs
        .downcast_iter()
        .zip(rhs.downcast_iter())
        .map(|(lat_arr, lon_arr)| {
            let mut mutarr = MutablePlString::with_capacity(lat_arr.len());

            for (lat_opt_val, lon_opt_val) in lat_arr.iter().zip(lon_arr.iter()) {
                match (lat_opt_val, lon_opt_val) {
                    (Some(lat_val), Some(lon_val)) => {
                        let res = &geocoder.search((*lat_val, *lon_val)).record.name;
                        mutarr.push(Some(res))
                    }
                    _ => mutarr.push_null(),
                }
            }

            mutarr.freeze().boxed()
        })
        .collect();
    let out: StringChunked = unsafe { ChunkedArray::from_chunks("placeholder", chunks) };
    Ok(out.into_series())
}

Simplified version with binary_elementwise_into_string_amortized:

#[polars_expr(output_type=String)]
fn reverse_geocode(inputs: &[Series]) -> PolarsResult<Series> {
    let lat = inputs[0].f64()?;
    let lon = inputs[1].f64()?;
    let geocoder = ReverseGeocoder::new();


    let out: StringChunked =
        binary_elementwise_into_string_amortized(lat, lon, |lhs, rhs, output| {
            let _ = write!(output, "{}", &geocoder.search((lhs, rhs)).record.name);
        });

    Ok(out.into_series())
}

[MarcoGorelli]

thanks @condekind - yup, this is exactly what I was suggesting, nice one

The function's currently unused in Polars - to prevent someone accidentally removing it, perhaps add a little docstring to it, in which you mention that it's intended as a utility for plugin authors?

[condekind]

Closing this since I didn't realize I opened the PR from the main branch. Will reopen from a dedicated branch and add the suggested docstrings, thank you @MarcoGorelli!