Add ChainHash type

The Lightning network defines a type called 'chain hash' that is used to
uniquely represent the various Bitcoin networks as a 32 byte hash value.
Chain hash is now being used by the DLC folks, as such it is useful to
have it implemented in rust-bitcoin.

One method of calculating a chain hash is by hashing the genesis block
for the respective network.

Add a `ChainHash` type that can be used to get the unique identifier of
each of the 4 Bitcoin networks we support. Add a method that returns
the chain hash for a network using the double sha256 of the genesis
block. Do so using hard coded consts and add unit
tests (regression/sanity) that show these hard code byte arrays match
the hash of the data we return for the genesis block for the respective
network.

The chain hash for the main Bitcoin network can be verified from LN
docs (BOLT 0), add a link to this document.

src/blockdata/constants.rs

@@ -176,6 +176,33 @@ pub fn genesis_block(network: Network) -> Block {
     }
 }
 
+// Mainnet value can be verified at https://github.com/lightning/bolts/blob/master/00-introduction.md
+const GENESIS_BLOCK_HASH_BITCOIN: [u8; 32] = [111, 226, 140, 10, 182, 241, 179, 114, 193, 166, 162, 70, 174, 99, 247, 79, 147, 30, 131, 101, 225, 90, 8, 156, 104, 214, 25, 0, 0, 0, 0, 0];
+const GENESIS_BLOCK_HASH_TESTNET: [u8; 32] = [67, 73, 127, 215, 248, 38, 149, 113, 8, 244, 163, 15, 217, 206, 195, 174, 186, 121, 151, 32, 132, 233, 14, 173, 1, 234, 51, 9, 0, 0, 0, 0];
+const GENESIS_BLOCK_HASH_SIGNET: [u8; 32] = [246, 30, 238, 59, 99, 163, 128, 164, 119, 160, 99, 175, 50, 178, 187, 201, 124, 159, 249, 240, 31, 44, 66, 37, 233, 115, 152, 129, 8, 0, 0, 0];
+const GENESIS_BLOCK_HASH_REGTEST: [u8; 32] = [6, 34, 110, 70, 17, 26, 11, 89, 202, 175, 18, 96, 67, 235, 91, 191, 40, 195, 79, 58, 94, 51, 42, 31, 199, 178, 183, 60, 241, 136, 145, 15];
+
+/// The uniquely identifying hash of the target blockchain.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct ChainHash([u8; 32]);
+impl_array_newtype!(ChainHash, u8, 32);
+impl_bytes_newtype!(ChainHash, 32);
+
+impl ChainHash {
+    /// Returns the hash of the `network` genesis block for use as a chain hash.
+    ///
+    /// See [BOLT 0](https://github.com/lightning/bolts/blob/ffeece3dab1c52efdb9b53ae476539320fa44938/00-introduction.md#chain_hash)
+    /// for specification.
+    pub fn using_genesis_block(network: Network) -> Self {
+        match network {
+            Network::Bitcoin => ChainHash(GENESIS_BLOCK_HASH_BITCOIN),
+            Network::Testnet => ChainHash(GENESIS_BLOCK_HASH_TESTNET),
+            Network::Signet => ChainHash(GENESIS_BLOCK_HASH_SIGNET),
+            Network::Regtest => ChainHash(GENESIS_BLOCK_HASH_REGTEST),
+        }
+    }
+}
+
 #[cfg(test)]
 mod test {
     use core::default::Default;
@@ -248,5 +275,49 @@ mod test {
         assert_eq!(format!("{:x}", gen.header.block_hash()),
                    "00000008819873e925422c1ff0f99f7cc9bbb232af63a077a480a3633bee1ef6".to_string());
     }
+
+    // The *_chain_hash tests are sanity/regression tests, they verify that the const byte array
+    // representing the genesis block is the same as that created by hashing the genesis block.
+    fn chain_hash_and_genesis_block(network: Network) {
+        use hashes::{sha256, Hash};
+
+        // The genesis block hash is a double-sha256 and it is displayed backwards.
+        let genesis_hash = genesis_block(network).block_hash();
+        // We abuse the sha256 hash here so we get a LowerHex impl that does not print the hex backwards.
+        let hash = sha256::Hash::from_slice(&genesis_hash.into_inner()).unwrap();
+        let want = format!("{:02x}", hash);
+
+        let chain_hash = ChainHash::using_genesis_block(network);
+        let got = format!("{:02x}", chain_hash);
+
+        // Compare strings because the spec specifically states how the chain hash must encode to hex.
+        assert_eq!(got, want);
+    }
+
+    macro_rules! chain_hash_genesis_block {
+        ($($test_name:ident, $network:expr);* $(;)*) => {
+            $(
+                #[test]
+                fn $test_name() {
+                    chain_hash_and_genesis_block($network);
+                }
+            )*
+        }
+    }
+
+    chain_hash_genesis_block! {
+        mainnet_chain_hash_genesis_block, Network::Bitcoin;
+        testnet_chain_hash_genesis_block, Network::Testnet;
+        signet_chain_hash_genesis_block, Network::Signet;
+        regtest_chain_hash_genesis_block, Network::Regtest;
+    }
+
+    // Test vector taken from: https://github.com/lightning/bolts/blob/master/00-introduction.md
+    #[test]
+    fn mainnet_chain_hash_test_vector() {
+        let got = format!("{:x}", ChainHash::using_genesis_block(Network::Bitcoin));
+        let want = "6fe28c0ab6f1b372c1a6a246ae63f74f931e8365e15a089c68d6190000000000";
+        assert_eq!(got, want);
+    }
 }