Function verify_kphf 

pub fn verify_kphf<K: Display, G: Fn(&K) -> Option<usize>>(
    k: u8,
    expected_range: usize,
    keys: impl IntoIterator<Item = K>,
    kphf: G,
)

Checks if kphf is valid partial k-perfect hash function. Panics if it is not (also if phf returns None for any key).