Add ruleset.rules accessor

[reid-rigo]

Hey @hmarr, really appreciate your work on this. I'm working on some Ruby bindings to this library, and this little tweak would be helpful to provide an accessor to the rules. I'm not experienced with Rust, so please advise if you think this should be done a different way, like via a getter.

[hmarr]

Thanks! You're right: getter would be preferable here. The constructor uses the rules vec to build the Matcher, and this change would mean that if you have a mutable reference to a RuleSet you'd be able to mutate the rules without rebuilding the Matcher.

Something like this ought to do it. This returns a reference to a slice of the rules rather than the mutable vector.

    pub fn rules(&self) -> &[Rule] {
        &self.rules
    }

I actually wrote Ruby bindings for this library a while back. It's in a private project owned by my ex-employer so I don't have access, but let me know if you need a hand.

[reid-rigo]

All right, I've updated the PR to use a function. Thanks for the feedback and feel free to make edits to your liking.

Awesome to hear that you've built Ruby bindings to this. We have an internal codeowners library built in Ruby, but it's too slow to use for some live purposes. I'm using magnus to make a gem and I'd love to get your feedback when I find the time to get things working.