Make RwLockReadGuard covariant over its type parameter

[Kestrer]

Currently RwLockReadGuard is invariant over its type parameter, meaning that for instance this sound code:

use parking_lot::{RwLock, RwLockReadGuard};

fn do_stuff<'a>(_: RwLockReadGuard<'_, &'a i32>, _: &'a i32) {}

fn main() {
    let j: i32 = 5;
    let lock = RwLock::new(&j);
    {
        let i = 6;
        do_stuff(lock.read(), &i);
    }
    drop(lock);
}

Will not compile. However, since RwLockReadGuard does not allow for mutation of the inner value, it would be sound to make it covariant, and it would be useful in some circumstances.

See also the same issue in std.