Function expand_key 

pub fn expand_key<const N: usize>(key: &[u8]) -> [[u8; 16]; N]

Expand a key into AES round keys (FIPS 197 §5.2).

• N = 11 -> AES-128 (Nk=4, Nr=10, 44 words -> 11 round keys)
• N = 15 -> AES-256 (Nk=8, Nr=14, 60 words -> 15 round keys)