## Issue Addressed
NA
## Proposed Changes
Adds additional tree hash caching for `state.eth1_data_votes`.
Presently, each time we tree hash the `BeaconState`, we recompute the `state.eth1_data_votes` tree in it's entirety. This is because we only previous had support for caching fixed-length lists.
This PR adds the `Eth1DataVotesTreeHashCache` which provides caching for the `state.eth1_data_votes` list. The cache is aware of `SLOTS_PER_ETH1_VOTING_PERIOD` and will reset itself whenever that boundary is crossed.
This cache adds a new (but somewhat fundamental) restriction to tree hash caching:
*For some state `s`, `s.tree_hash_cache` is only valid for `s` or descendants of `s` that have been reached via state transitions that are faithful to the specification (invalid blocks are permitted, as long as they are faithfully processed).*
21bcc8848d