damus-c: add bolt11 parser from CLN

We'll need this for our lightning invoice view

Signed-off-by: William Casarin <jb55@jb55.com>

damus-c/utf8.h

@@ -51,4 +51,7 @@ bool utf8_decode(struct utf8_state *utf8_state, char c);
  * Sets errno to ERANGE if point was invalid.
  */
 size_t utf8_encode(uint32_t point, char dest[UTF8_MAX_LEN]);
+
+/* Check for valid UTF-8 */
+bool utf8_check(const void *vbuf, size_t buflen);
 #endif /* CCAN_UTF8_H */