These use onion encoding for simple one-way messaging: there are no error returns.
However, every onion uses route blinding *even if it doesn't need to*.

You can prove what path was used to reach you by including `path_id` in the
encrypted_data_tlv.

Note that this doesn't actually define the payload we're transporting:
that's explictly defined to be payloads in the 64-255 range.

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

Typo fixes from @t-bast, @thomash-acinq and @remyers.

@thomash-acinq points out:

1. We absolutely can put other fields in `encrypted_data_tlv`, esp. padding, and test vectors do this.
2. Presumably it was supposed to refer to onionmsg_tlv, so fix that.
3. And of course we need to allow payload fields!