* Rename RootHash to Commitment based on ZIP-244
Interactive replace using:
```sh
fastmod RootHash Commitment
fastmod root_hash commitment
fastmod root_bytes commitment_bytes
git mv zebra-chain/src/block/root_hash.rs zebra-chain/src/block/commitment.rs
```
All replacements were accepted.
* rustfmt
* Comment and format cleanups after interactive replace
* Distinguish Sapling tree roots from other tree roots
* Add the NU5 BlockCommitmentsHash variant to block::Commitment
This change parses the hash, but does not perform validation.
* Validate reserved values in Block::commitment
- change Block::commitment to return a Result rather than an Option
- enforce the all-zeroes reserved value consensus rules
- change `PreSaplingReserved([u8; 32])` to `PreSaplingReserved`
- change `ChainHistoryActivationReserved([u8; 32])` to `ChainHistoryActivationReserved`
- update the function comments to describe when each variant is verified
* Fix comment whitespace
teor