impl FromStr for BlockHeaderHash

This is very convenient for putting block hashes in the source code.

zebra-chain/src/block.rs

@@ -67,6 +67,18 @@ impl ZcashDeserialize for BlockHeaderHash {
     }
 }
 
+impl std::str::FromStr for BlockHeaderHash {
+    type Err = SerializationError;
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let mut bytes = [0; 32];
+        if hex::decode_to_slice(s, &mut bytes[..]).is_err() {
+            Err(SerializationError::Parse("hex decoding error"))
+        } else {
+            Ok(BlockHeaderHash(bytes))
+        }
+    }
+}
+
 /// Block header.
 ///
 /// How are blocks chained together? They are chained together via the