Add NextSlotsIterator (#8652)

automerge

ledger/src/lib.rs

@@ -12,6 +12,7 @@ pub mod genesis_utils;
 pub mod leader_schedule;
 pub mod leader_schedule_cache;
 pub mod leader_schedule_utils;
+pub mod next_slots_iterator;
 pub mod poh;
 pub mod rooted_slot_iterator;
 pub mod shred;