Casper circuit: finality calculation

To implement (in-circuit) finality calculations to justify/finalize `target_epoch`. This involves:
1. calculating the `total_active_balance` of all _active_ validators.
2. calculating the `total_effective_balance` of all _attested_ validators.
3. checking that `total_effective_balance >= 2/3 * total_active_balance`

### [Spec](https://hackmd.io/@timofey/rks1Y7BL3)

### References
- https://github.com/ChainSafe/Zipline/blob/main/zipline-verifier/src/verify.rs#L163