2d8c613340 Move the block hash types to primitives (Tobin C. Harding)
6b9429ac7b Remove BlockHash::all_zeros (Tobin C. Harding)
20d8dbd586 Add missing line of whitespace (Tobin C. Harding)
Pull request description:
As an initial step in moving the `block` module, just move over the hash types `BlockHash` and `WitnessCommitment`.
Patch 2 introduces an associated const `BlockHash::GENESIS_PREV_BLOCKHASH` and removes `all_zeros`.
ACKs for top commit:
apoelstra:
ACK 2d8c613340 successfully ran local tests
Tree-SHA512: 64aa0ae81e1c8ab1b5d4cd8cd28e6ef04ed01bf79175dc5b1fd607a6f0967e06b0aaf4c10ad368e2b327edcad3705187b6643d5ca8647716319424f19a838ba1