Implement ArbitraryOrd for relative::LockTime

TL;DR As we do for `absolute::LockTime` and for the same reasons;
implement `ArbitraryOrd` for `relative::LockTime`.

locktimes do not have a semantic ordering if they differ (blocks, time)
so we do not derive `Ord` however it is useful for downstream to be able
to order structs that contain lock times. This is exactly what the
`ArbitraryOrd` trait is for.

Fix: rust-bitcoin#2566

bitcoin/src/blockdata/locktime/relative.rs

@@ -6,6 +6,8 @@
 //! whether bit 22 of the `u32` consensus value is set.
 //!
 
+#[cfg(feature = "ordered")]
+use core::cmp::Ordering;
 use core::fmt;
 
 #[cfg(all(test, mutate))]
@@ -191,6 +193,20 @@ impl fmt::Display for LockTime {
     }
 }
 
+#[cfg(feature = "ordered")]
+impl ordered::ArbitraryOrd for LockTime {
+    fn arbitrary_cmp(&self, other: &Self) -> Ordering {
+        use LockTime::*;
+
+        match (self, other) {
+            (Blocks(_), Time(_)) => Ordering::Less,
+            (Time(_), Blocks(_)) => Ordering::Greater,
+            (Blocks(this), Blocks(that)) => this.cmp(that),
+            (Time(this), Time(that)) => this.cmp(that),
+        }
+    }
+}
+
 /// Tried to satisfy a lock-by-blocktime lock using a height value.
 #[derive(Debug, Clone, PartialEq, Eq)]
 #[non_exhaustive]