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.c

@@ -1,6 +1,7 @@
 
 #include "cursor.h"
 #include "invoice.h"
+#include "nostrdb.h"
 #include "bolt11/bolt11.h"
 #include "bolt11/amount.h"