nostrdb/blocks: add note block iterator

This adds an api that walks along and pulls compact note block data out of
nostrdb.

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

nostrdb/src/str_block.h

@@ -4,7 +4,7 @@
 
 #include <inttypes.h>
 
-struct str_block {
+struct ndb_str_block {
 	const char *str;
 	uint32_t len;
 };