feat: add PathSeg to public API

[geoffreygarrett]

Rationale:

I'm doing a bit of code generation from the parsed template, and in order to generate based on:

Path(
    Relative(
        ( 
            [Named("visibility")],
            "visibility",
        ),
    ),
),

I've been using the following workaround, which is far from ideal:

fn path_to_string(path: &Path) -> String {
    let raw_path = format!("{:?}", path);
    let regex = regex::Regex::new(r#"(?:"|\\")([^\\"]+)(?:"|\\")"#).unwrap();
    if let Some(captures) = regex.captures(&raw_path) {
        captures.get(1).unwrap().as_str().to_string()
    } else {
        raw_path
    }
}

Making PathSeg part of the public API allows for cleaner and more direct usage, eliminating the need for such workarounds.

Thank you for your consideration.

[coveralls]

coverage: 81.191%. remained the same
when pulling 694b980 on geoffreygarrett:master
into 57454fc on sunng87:master.

[sunng87]

Thank you! Could you please add #[non_exhaustive] to PathSeg as well? We need that attribute for public members.

[coveralls]

coverage: 81.457% (+0.3%) from 81.191%
when pulling c3a4e5f on geoffreygarrett:master
into 57454fc on sunng87:master.