Rather than keeping each hop, we can generate it in place since we only
need the first hop result.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This means we can save the partial HMAC of the padding for each step,
rather than the padding itself, when generating it.
Each step now takes the *last*, not *first* part of the onion array.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
