chain: add custom Debug for CoinbaseData

The derived Debug impl just shows u8s as numbers, which isn't what we
want.  There are basically two reasonable options here:

1. Hex-encoded bytes
2. Escaped ASCII

I picked (2) because a lot of coinbase data has ascii text in it.

zebra-chain/src/transparent.rs

@@ -23,7 +23,7 @@ use crate::{
 };
 
 /// Arbitrary data inserted by miners into a coinbase transaction.
-#[derive(Clone, Debug, Eq, PartialEq, Serialize, Deserialize)]
+#[derive(Clone, Eq, PartialEq, Serialize, Deserialize)]
 pub struct CoinbaseData(
     /// Invariant: this vec, together with the coinbase height, must be less than
     /// 100 bytes. We enforce this by only constructing CoinbaseData fields by
@@ -40,6 +40,20 @@ impl AsRef<[u8]> for CoinbaseData {
     }
 }
 
+impl std::fmt::Debug for CoinbaseData {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let escaped = String::from_utf8(
+            self.0
+                .iter()
+                .cloned()
+                .flat_map(std::ascii::escape_default)
+                .collect(),
+        )
+        .expect("ascii::escape_default produces utf8");
+        f.debug_tuple("CoinbaseData").field(&escaped).finish()
+    }
+}
+
 /// OutPoint
 ///
 /// A particular transaction output reference.