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>

common/onionreply.h

@@ -0,0 +1,24 @@
+#ifndef LIGHTNING_COMMON_ONIONREPLY_H
+#define LIGHTNING_COMMON_ONIONREPLY_H
+#include "config.h"
+#include <ccan/short_types/short_types.h>
+#include <ccan/tal/tal.h>
+
+/* A separate type for an onion reply, to differentiate from a wire msg. */
+struct onionreply {
+	u8 *contents;
+};
+
+/**
+ * Wire marshalling routines for onionreply
+ */
+void towire_onionreply(u8 **cursor, const struct onionreply *r);
+struct onionreply *fromwire_onionreply(const tal_t *ctx,
+				       const u8 **cursor, size_t *max);
+
+
+struct onionreply *dup_onionreply(const tal_t *ctx,
+				  const struct onionreply *r TAKES);
+
+struct onionreply *new_onionreply(const tal_t *ctx, const u8 *contents TAKES);
+#endif /* LIGHTNING_COMMON_ONIONREPLY_H */