Function for_hashmap_key 

pub fn for_hashmap_key<K, V>(
    key: K,
) -> OptionalKeyPath<HashMap<K, V>, V, impl for<'r> Fn(&'r HashMap<K, V>) -> Option<&'r V>>
where
    K: Hash + Eq + Clone + 'static,
    V: 'static,

Create a keypath for key-based access in HashMap<K, V>