Add a method to check if all Predicates in some VersionReq contains some Op

[hbina]

Hello,

I am trying to resolve this issue here : rust-lang/cargo#5340
But to do that --- without resorting to copying the behavior of VersionReq --- I would need an API that can easily let me check if all of my Predicates in some VersionReq matches some Op.

I would like something like this to be possible,

(semver::VersionReq).contains_op(semver::Op).

I believe this will require couple of changes:

1. Op is now a public enum
2. Add a new method contains_op that takes an Op and returns a bool as described above.

Thank you!

[dtolnay]

As of 1.0.0, this logic is now possible to implement downstream, and doesn't need to be built into this crate.

use semver::{Op, VersionReq};

fn contains_op(req: &VersionReq, op: Op) -> bool {
    req.comparators.iter().all(|cmp| cmp.op == op)
}