Function pk_decode 

pub fn pk_decode<P: Params>(pk: &[u8]) -> ([u8; 32], [[i32; 256]; 8])

Decode a public key from bytes.

Implements Algorithm 23 of FIPS 204 (pkDecode). Inverse of pk_encode.

Returns (rho, t1) where rho is the 32-byte seed and t1 is the fixed array of k polynomials with coefficients in [0, 1023].