Function sk_decode 

pub fn sk_decode<P: Params>(
    sk: &[u8],
) -> ([u8; 32], [u8; 32], [u8; 64], [[i32; 256]; 7], [[i32; 256]; 8], [[i32; 256]; 8])

Decode a secret key from bytes.

Implements Algorithm 25 of FIPS 204 (skDecode). Inverse of sk_encode.

Returns (rho, K, tr, s1, s2, t0) using fixed arrays.