Function replace_all 

pub fn replace_all(
    pattern: &FindPattern,
    lines: &[String],
    replacement: &str,
) -> Option<(Vec<String>, usize)>

Rewrite every match in lines, returning the new lines and how many matches were rewritten. The replace all primitive.

Returns None when nothing matched, so callers can distinguish “no work” from “rewrote zero characters” without re-counting.