refactor: move conversion to string-duration fully to R

[etiennebacher]

Currently, parsing duration is done like that:

• pass the string from R to Rust
• eval parse_as_polars_duration_string() from Rust using the R! macro
• use this result in Rust

Since parse_as_polars_duration_string() is already defined in R, we could just call this on the string and then pass the result to Rust.

Also added a couple of small refactor and clippy suggestions.

[eitsupi]

Thanks!