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.c

@@ -178,3 +178,22 @@ size_t utf8_encode(uint32_t point, char dest[UTF8_MAX_LEN])
     return 4;
 }
 
+/* Check for valid UTF-8 */
+bool utf8_check(const void *vbuf, size_t buflen)
+{
+    const unsigned char *buf = vbuf;
+    struct utf8_state utf8_state = UTF8_STATE_INIT;
+    bool need_more = false;
+
+    for (size_t i = 0; i < buflen; i++) {
+        if (!utf8_decode(&utf8_state, buf[i])) {
+            need_more = true;
+            continue;
+        }
+        need_more = false;
+        if (errno != 0)
+            return false;
+    }
+    return !need_more;
+}
+