## 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.
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