Manual Antichain::default to avoid bounds (#537)

The derived `default` for Antichain includes a bound `T: Default`, which is
unnecessary. This change replaces the derived implementation with a custom
one that does not have the constrain.

Signed-off-by: Moritz Hoffmann <antiguru@gmail.com>

Author: antiguru

timely/src/progress/frontier.rs

@@ -13,7 +13,7 @@ use crate::order::{PartialOrder, TotalOrder};
 /// Two antichains are equal if the contain the same set of elements, even if in different orders.
 /// This can make equality testing quadratic, though linear in the common case that the sequences
 /// are identical.
-#[derive(Debug, Default, Abomonation, Serialize, Deserialize)]
+#[derive(Debug, Abomonation, Serialize, Deserialize)]
 pub struct Antichain<T> {
     elements: Vec<T>
 }
@@ -264,6 +264,12 @@ impl<T: Clone> Clone for Antichain<T> {
     }
 }
 
+impl<T> Default for Antichain<T> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl<T: TotalOrder> TotalOrder for Antichain<T> { }
 
 impl<T: TotalOrder> Antichain<T> {