ed80df5ebc Add `ChainHash::from_genesis_block_hash` (Martin Habovstiak)
Pull request description:
This improves readability of converting `BlockHash` into `ChainHash`. It's useful in e.g. Electrum protocol which sends `BlockHash` (serialized backward).
Closes#1751
ACKs for top commit:
apoelstra:
ACK ed80df5ebc
tcharding:
ACK ed80df5ebc
Tree-SHA512: b03eb9ca0a1eb38196cc8e43751c2def0d00b463b774003c5319bf56599db598ddd4bb0e2ad0bd3803f21a925135992d421848e7b1ba5e7bab3a405f60fc973c