Function derive_key 

pub fn derive_key<H: Hasher, const N: usize>(
    ikm: &[u8],
    info: &[u8],
    salt: Option<&[u8]>,
) -> Result<[u8; N], HkdfError>

One-shot extract-then-expand.

Error

Returns an error if if N > 255 * H::OUTPUT_SIZE.