Add PartialOrd and Ord for PeerId (#1594)

for using PeerId in order based collections. Ord uses borrow to be
consistent with equality.

core/src/peer_id.rs

@@ -23,7 +23,7 @@ use bs58;
 use thiserror::Error;
 use multihash::{self, Code, Sha2_256};
 use rand::Rng;
-use std::{convert::TryFrom, borrow::Borrow, fmt, hash, str::FromStr};
+use std::{convert::TryFrom, borrow::Borrow, fmt, hash, str::FromStr, cmp};
 
 /// Public keys with byte-lengths smaller than `MAX_INLINE_KEY_LENGTH` will be
 /// automatically used as the peer id using an identity multihash.
@@ -57,6 +57,21 @@ impl fmt::Display for PeerId {
     }
 }
 
+impl cmp::PartialOrd for PeerId {
+    fn partial_cmp(&self, other: &Self) -> Option<cmp::Ordering> {
+        Some(Ord::cmp(self, other))
+    }
+}
+
+impl cmp::Ord for PeerId {
+    fn cmp(&self, other: &Self) -> cmp::Ordering {
+        // must use borrow, because as_bytes is not consistent with equality
+        let lhs: &[u8] = self.borrow();
+        let rhs: &[u8] = other.borrow();
+        lhs.cmp(rhs)
+    }
+}
+
 impl PeerId {
     /// Builds a `PeerId` from a public key.
     pub fn from_public_key(key: PublicKey) -> PeerId {