Add an API for reading files from the RPM payload

[dralley]

📜 Checklist

• Commits are cleanly separated and have useful messages
• A changelog entry or entries has been added to CHANGELOG.md
• Documentation is thorough
• Test coverage is excellent and passes
• Works when tests are run --all-features enabled

Note: I'm working on tests, but they will be easier to merge as part of the test overhaul PR. In the meantime I did some manual testing for this PR.

[dralley]

Here's a bit of example code for demo purposes:

use rpm;
use std::env::args;

fn main() {
    let rpm_path = args().nth(1).expect("provide an RPM to extract");
    let package =
    rpm::Package::open(rpm_path).unwrap();

    let dest_path = args().nth(2).unwrap_or(package.metadata.get_name().unwrap().to_owned());

    package.extract(&dest_path).unwrap();

    for f in package.files().unwrap() {
        let f = f.unwrap();
        println!("{}", f.metadata.path.display());
    }
}

[drahnr]

Looks good, bar some more documentation details

[dralley]

@drahnr I tweaked the behavior slightly such that it uses create_dir instead of create_dir_all. That simplifies the semantics.