Function parse_all 

pub fn parse_all<I, S>(arguments: I) -> Result<Vec<Step>, VerbError>
where
    I: IntoIterator<Item = S>,
    S: AsRef<str>,

Parse every argument, or fail on the first bad one.

All-or-nothing by design: a chain with a typo in step 7 must not perform steps 1 through 6 first.