common/blindedpay: routines to construct a blinded payment.

Don't shoehorn it into onion_nonfinal_hop() and onion_final_hop(), but
provide an explicit routine "blinded_onion_hops" and an onion helper
"onion_blinded_hop()" for it to call.

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

devtools/onion.c

@@ -79,15 +79,12 @@ static void do_generate(int argc, char **argv,
 				sphinx_add_hop_has_length(sp, &path[i],
 					       take(onion_final_hop(NULL,
 								    amt, i, amt,
-								    NULL, NULL,
 								    NULL, NULL)));
 			else
 				sphinx_add_hop_has_length(sp, &path[i],
 					       take(onion_nonfinal_hop(NULL,
 								       &scid,
-								       amt, i,
-								       NULL,
-								       NULL)));
+								       amt, i)));
 		}
 	}