Also update the fuzz ChaCha20Poly1305 to not mark as finished after a single
encrypt_in_place. This is because more bytes may still need to be encrypted,
causing us to panic at the assertion that finished == false when we go to
encrypt more.
Also fix unused_mut warning in messenger + add log on OM forward for testing
This required adapting `onion_utils::decode_next_hop` to work for both payments
and onion messages.
Currently we just print out the path_id of any onion messages we receive. In
the future, these received onion messages will be redirected to their
respective handlers: i.e. an invoice_request will go to an InvoiceHandler,
custom onion messages will go to a custom handler, etc.
This adds several utilities in service of then adding
OnionMessenger::send_onion_message, which can send to either an unblinded
pubkey or a blinded route. Sending custom TLVs and sending an onion message
containing a reply path are not yet supported.
We also need to split the construct_keys_callback macro into two macros to
avoid an unused assignment warning.
