Function expand 

pub fn expand<H: Hasher, const N: usize>(
    prk: &[u8],
    info: &[u8],
) -> Result<[u8; N], HkdfError>

Expand step: OKM = T(1) || T(2) || ..., where T(i) = HMAC-Hash(PRK, T(i-1) || info || i).

Error

Returns an error if N > 255 * H::OUTPUT_SIZE or if prk.len() < H::OUTPUT_SIZE.