Function write_joined_with 

pub fn write_joined_with<W, I, F>(
    writer: &mut W,
    values: I,
    separator: &str,
    write_value: F,
) -> Result<(), Error>
where
    W: Write + ?Sized,
    I: IntoIterator,
    F: FnMut(&mut W, <I as IntoIterator>::Item) -> Result<(), Error>,

Write values separated by separator, using write_value for each value.

Nothing is allocated, and the separator is written only between values.