debug: improve Debug impl for Ed25519 VerificationKeys

Since the `ed25519_consensus::VerificationKey` is no longer used, the
`VerificationKey`s go back to having raw debug output that's hard to read when
it appears in logs.  This commit restores the hex-formatting previously
inherited from `ed25519_consensus`, allowing nice output like (in use in `pd`
downstream):

```
2023-02-24T03:34:57.002766Z DEBUG abci:EndBlock{height=187}:end_block:staking:build_tendermint_validator_updates: updates=[Update { pub_key: Ed25519(7423aa71eb844cdde4dc0b8c72ef1225931dc54c21a102cbe2fded2645d4e814), power: Power(50000000000) }]
```

In the future, another option could be to have the key be base64-encoded, to
match Tendermint's data structures, but this is a less obtrusive change (not
requiring a base64 encoder) to restore functionality first.

.changelog/unreleased/bug-fixes/1272-ed25519-debug.md

@@ -0,0 +1,2 @@
+- `[tendermint]` Restore hex-formatting in debug output of Ed25519 keys.
+  ([#1272](https://github.com/informalsystems/tendermint-rs/pull/1272))

tendermint/src/crypto/ed25519/verification_key.rs

@@ -1,8 +1,23 @@
 use crate::Error;
 
-#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+#[derive(Copy, Clone, Eq, PartialEq)]
 pub struct VerificationKey([u8; 32]);
 
+impl core::fmt::Display for VerificationKey {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        for byte in &self.0 {
+            write!(f, "{byte:02x}")?;
+        }
+        Ok(())
+    }
+}
+
+impl core::fmt::Debug for VerificationKey {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        <Self as core::fmt::Display>::fmt(self, f)
+    }
+}
+
 impl VerificationKey {
     #[allow(dead_code)]
     pub(super) fn new(bytes: [u8; 32]) -> Self {