Use forwards iterator for state root lookups (#2422)

## Issue Addressed

#2377 

## Proposed Changes

Implement the same code used for block root lookups (from #2376) to state root lookups in order to improve performance and reduce associated memory spikes (e.g. from certain HTTP API requests).

## Additional Changes

- Tests using `rev_iter_state_roots` and `rev_iter_block_roots` have been refactored to use their `forwards` versions instead.
- The `rev_iter_state_roots` and `rev_iter_block_roots` functions are now unused and have been removed.
- The `state_at_slot` function has been changed to use the `forwards` iterator.

## Additional Info

- Some tests still need to be refactored to use their `forwards_iter` versions. These tests start their iteration from a specific beacon state and thus use the `rev_iter_state_roots_from` and `rev_iter_block_roots_from` functions. If they can be refactored, those functions can also be removed.

beacon_node/http_api/tests/tests.rs

@@ -861,13 +861,10 @@ impl ApiTester {
     pub async fn test_beacon_headers_all_parents(self) -> Self {
         let mut roots = self
             .chain
-            .rev_iter_block_roots()
+            .forwards_iter_block_roots(Slot::new(0))
             .unwrap()
             .map(Result::unwrap)
             .map(|(root, _slot)| root)
-            .collect::<Vec<_>>()
-            .into_iter()
-            .rev()
             .collect::<Vec<_>>();
 
         // The iterator natively returns duplicate roots for skipped slots.