Function all_btree_map_keys 

pub fn all_btree_map_keys<K, V>(
    predicate: fn(&K) -> bool,
) -> impl Fn(&BTreeMap<K, V>) -> bool

Checks if all keys in the BTreeMap satisfy the predicate.

#[derive(Debug, Valust)]
struct All {
    #[valid(func(all_btree_map_keys(|&k| k > 1)))]
    data: BTreeMap<u8, u8>
}

let all = Raw::<All> { data: vec![(1, 2), (2, 3), (3, 4)].into_iter().collect() };
let val = All::validate(all);
assert!(val.is_err());
println!("{}", val.unwrap_err().full_stringify());