Map `get_key_value` implementation before Rust v1.40.0

map.rs

// On stable: <https://doc.rust-lang.org/std/collections/struct.HashMap.html#method.get_key_value>
pub fn get_key_value<'a,K,V,Q: ?Sized>(map: &'a HashMap<K,V>, k: &Q) -> Option<(&'a K, &'a V)>
    where K: std::borrow::Borrow<Q> + std::hash::Hash + std::cmp::Eq,
        Q: std::hash::Hash + Eq {
    let key = map.keys().find(|key| key.borrow() == k)?;
    Some((key, map.get(key.borrow())?))
}