common: add struct onionreply

I really want a type which means "I am a wrapped onion reply" as separate
from "I am a normal wire msg".  Currently both user u8 *, and I got very
confused trying to figure out where each one was an unwrapped error msg,
or where it still needed (un)wrapping.

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

tools/generate-wire.py

@@ -224,6 +224,7 @@ class Type(FieldSet):
         'bitcoin_tx_output',
         'exclude_entry',
         'fee_states',
+        'onionreply',
     ]
 
     # Some BOLT types are re-typed based on their field name