bitcoin: Add comparison between pubkeys

Some of the routing messages rely on a canonical ordering of pubkeys.

bitcoin/pubkey.c

@@ -65,3 +65,11 @@ bool pubkey_eq(const struct pubkey *a, const struct pubkey *b)
 {
 	return structeq(&a->pubkey, &b->pubkey);
 }
+
+int pubkey_cmp(const struct pubkey *a, const struct pubkey *b)
+{
+	u8 keya[33], keyb[33];
+	pubkey_to_der(keya, a);
+	pubkey_to_der(keyb, b);
+	return memcmp(keya, keyb, sizeof(keya));
+}

bitcoin/pubkey.h

@@ -32,4 +32,7 @@ void pubkey_to_der(u8 der[PUBKEY_DER_LEN], const struct pubkey *key);
 
 /* Are these keys equal? */
 bool pubkey_eq(const struct pubkey *a, const struct pubkey *b);
+
+/* Compare the keys `a` and `b`. Return <0 if `a`<`b`, 0 if equal and >0 otherwise */
+int pubkey_cmp(const struct pubkey *a, const struct pubkey *b);
 #endif /* LIGHTNING_PUBKEY_H */