Added utility library for parsing workspace status stamps.

[UebelAndre]

This change introduces a small utility for parsing workspace status stamp files. I found similar snippets throughout projects I work on and thought consolidation within rules_rust would be good. An example usage can be seen here:

use workspace_status::parse_workspace_status_stamps;

fn main() {
    let stable_status = std::fs::read_to_string("bazel-out/stable-status.txt").unwrap();
    let volatile_status = std::fs::read_to_string("bazel-out/volatile-status.txt").unwrap();
    let stamps = parse_workspace_status_stamps(&stable_status)
                .chain(parse_workspace_status_stamps(&stable_status))
                .flatten()
                .collect::<BTreeMap<_, _>>();
    // ...
    // ...
    // ...
}

[illicitonion]

Seems useful! Thanks!