92ef41b663 Make `ChainHash::using_genesis_block` constant (Jeffrey Czyz)
Pull request description:
ChainHash::using_genesis_block can't be `const` if it uses a `match` expression prior to Rust 1.46. Use an array mapping to work around this limitation.
Follow-up suggested in [#1283](https://github.com/rust-bitcoin/rust-bitcoin/pull/1283#issuecomment-1249418809).
ACKs for top commit:
apoelstra:
ACK 92ef41b663
Kixunil:
ACK 92ef41b663
Tree-SHA512: 71f95877c8e5335012ad0339e1f8691e3b33344fa02ecc24c3d4d728232cb7b0de62aec20eb1855b23eeccfbc2eeab920b21ee2243d95c6c89fa8ad5bc846975