nostrdb/Inital embedded content parser

This adds some initial code for nostrdb content parsing.

We still need to write tests for encoding and decoding, so this is
likely not working yet.

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

nostrdb/src/invoice.h

@@ -2,6 +2,9 @@
 #ifndef NDB_INVOICE_H
 #define NDB_INVOICE_H
 
+#include <inttypes.h>
+#include "cursor.h"
+
 struct bolt11;
 
 struct ndb_invoice {