This function is used to get the merkle root of some block roots which are themselves hashes, so the leaves do not need to be rehashed.
Unfortunately, it's easy to assume that a merkle_root function would hash its leaves, so it should be clarified.
If this is accepted, then we can replace all uses of signing in the protocol, which are currently done in a relatively inconsistent way (see proposer signatures, attester signatures, shard proposer signatures, exit message signatures.....) could be unified.
* Bring forward changes to withdrawability from phase 1
* The `WITHDRAWABLE` flag is removed; instead, a validator's withdrawability is determined through the `withdrawable_epoch` field (renamed and re-purposed from `withdrawal_epoch` which was not used)
* When a validator passes through the withdrawal queue, the `prepare_validator_for_withdrawal` function does not let them withdraw immediately; instead, they have to wait `MIN_VALIDATOR_WITHDRAWAL_EPOCHS`. This extra minimum delay serves no value in phase 0, but is crucial for phase 1 as the period between a validator passing through the queue and the validator being eligible to withdraw is where proof of custody challenges can come in; adding it in phase 0 is only half a line of code so easier to add it now.
* If a validator is penalized, they are no longer subject to the exit queue; instead, their `withdrawable_epoch` is set `LATEST_PENALIZED_EXIT_LENGTH` into the future and this is used to determine when the validator can withdraw
* Changes the eligibility condition for a transfer to use the `withdrawable_epoch`
1. Use `+` to concatenate the merkle roots in `hash` function.
2. Fix `pad_to_power_of_2`: padding with `[b'\x00' * SHARD_BLOCK_SIZE]`,
not `[SHARD_BLOCK_SIZE]`.
Reasons to use `merkle_hash` instead of `hash` for containers:
1) **Smaller witnesses**: `BeaconState` is a somewhat wide container (26 fields as of now, likely 30+ in phase 2). With concatenation and plain concatenation the size of the Merkle witnesses for the top level are ~32 bytes per field element.
2) **Faster incremental hashing**
3) **Consistency**: Consistent with `merkle_hash` for lists/vectors.
