Add format_str to lazy expressions API

[hpux735]

To support other algorithm development, and to match python behavior, it was necessary to implement the format function. In Rust, because there's a macro named format! I thought it best to name it format_str to avoid confusion. This function should have similar time performance as concat_str, because it only scans the format string once, and it's implemented via concat_str. It should, also, be linear time.

[ritchie46]

Thanks. I have a small remark.