iter-identify_first_last

A helper iterator, flagging first and last elements.

```rust use iteridentifyfirst_last::IteratorIdentifyFirstLastExt; use std::fmt::Write;

fn main() { let list = [1, 2, 3, 4, 5, 6]; let mut string = String::new(); for (isfirst, item) in list.iter().identifyfirst() { if !isfirst { string.push(' '); } write!(string, "{item}").unwrap(); } asserteq!(string, "1 2 3 4 5 6"); } ```