Unimplement TreeHash for BeaconState

[michaelsproul]

Issue Addressed

Since we moved to tree-states the tree_hash_root method on BeaconState became potentially dangerous. It would panic if any pending updates were waiting to be applied to the tree. We've done a good job of vetting that this doesn't happen on production code paths, but it's easy to do accidentally in tests, and I'm worried about regressions.

More background here:

• Infallible immutable tree hash milhouse#43

Proposed Changes

• Remove the trait implementation of TreeHash for BeaconState as a safeguard against accidental misuse.
• Change the canonical_root method to call the safe mutable method update_tree_hash_cache.
• Update all usages (mostly tests). I've used unwrap liberally in tests as hashing a valid state should never fail.

[dapplion]

Quite painless change to remove this nasty footgun. Future mantainers will appreciate :)

[michaelsproul]

@mergify queue

[mergify]

queue

✅ The pull request has been merged automatically

The pull request has been merged automatically at 2f0af2b