Function sodalite::box_beforenm

pub fn box_beforenm(k: &mut [u8; 32], pk: &BoxPublicKey, sk: &BoxSecretKey)

By splitting box_ into 2 steps: box_beforenm and box_afternm, we can more efficiently compute multiple messages that use the same keys.

The k can be reused for any messages that would use the same public key pk and secret key sk.