Signed-off-by: Rusty Russell <[email protected]>

We usually assume we're fetching an invoice we are going to pay, so we
look up the previous payment for the payer key, and other sanity
checks.

This adds a developer option to fetchinvoice, which allows it to force
its own payer key, which it uses to sign directly and bypasses these
checks.

Signed-off-by: Rusty Russell <[email protected]>

And fix up the mess we'd made:
1. We didn't order merkles by lesser-first.
2. We didn't correctly construct tree with last nodes on shortest path.

Now we have tests!

Signed-off-by: Rusty Russell <[email protected]>
Changelog-EXPERIMENTAL: protocol: offer signature format changed.

The one about size needs the offers PR rebased onto the latest
onion-message PR.

Signed-off-by: Rusty Russell <[email protected]>

And add annotation so compiler spots it!

Signed-off-by: Rusty Russell <[email protected]>