This doesn't specify *how* they are encoded in the onion, since that's
still being discussed.

Currently it just defines switch_chain and switch_ephkey, which are
simple.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

I don't use the odd-is-OK rule, because we have many other ways to break
the format if we really want it.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Note the warning on non-amount MPP: unlike real goods with a receipt,
the recipient is not necessarily motivated to wait for the entire
payment (esp. if it's about to time out).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>