Function xmss_sign_into 

pub fn xmss_sign_into<P: Params>(
    m: &[u8],
    sk_seed: &[u8],
    idx: u32,
    pk_seed: &[u8],
    adrs: &mut Adrs,
    out: &mut [u8],
)

Streaming variant of xmss_sign — writes the (LEN + H') * N byte signature into the start of out (which must be at least that size) instead of returning a freshly-allocated Vec<u8>.

Layout of out (after the call): WOTS sig (LEN * N) || auth path (H' * N).