nostrdb/blocks: expose block iterator internals

so we don't need heap allocation. we will be calling this a lot in tight
render loops, we don't want to be allocating on each frame.

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

nostrdb/src/invoice.h

@@ -4,18 +4,10 @@
 
 #include <inttypes.h>
 #include "cursor.h"
+#include "nostrdb.h"
 
 struct bolt11;
 
-struct ndb_invoice {
-	unsigned char version;
-	uint64_t amount;
-	uint64_t timestamp;
-	uint64_t expiry;
-	char *description;
-	unsigned char *description_hash;
-};
-
 // ENCODING
 int ndb_encode_invoice(struct cursor *cur, struct bolt11 *invoice);
 int ndb_decode_invoice(struct cursor *cur, struct ndb_invoice *invoice);