/// A drop-in replacement for `std::format!`, which can optionally accept a an
/// existing `String` buffer.
///
/// ```rust
/// use format_buf::format;
///
/// let mut buf = format!("Roses are {},\n", "red");
/// let () = format!(buf, "Violets are {}.", "blue");
/// assert_eq!(buf, "\
/// Roses are red,\n\
/// Violets are blue.\
/// ")
/// ```