We used to have a permutation map; this reintroduces a variant which
uses the htlc pointers directly.
We need this because we have to send the htlc-tx signatures in output
order as part of the protocol: without two-stage HTLCs we only needed
to wire them up in the unilateral spend case so we simply brute-forced
the ordering.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>