delegation: Implement list delegation

```rust
reuse prefix::{a, b, c}
```

src/items.rs

@@ -739,8 +739,8 @@ fn visit_impl_items(&mut self, items: &[ptr::P<ast::AssocItem>]) {
                 (_, Const(..)) => Ordering::Greater,
                 (MacCall(..), _) => Ordering::Less,
                 (_, MacCall(..)) => Ordering::Greater,
-                (Delegation(..), _) => Ordering::Less,
-                (_, Delegation(..)) => Ordering::Greater,
+                (Delegation(..), _) | (DelegationMac(..), _) => Ordering::Less,
+                (_, Delegation(..)) | (_, DelegationMac(..)) => Ordering::Greater,
             });
             let mut prev_kind = None;
             for (buf, item) in buffer {

src/visitor.rs

@@ -586,7 +586,7 @@ pub(crate) fn visit_item(&mut self, item: &ast::Item) {
                     );
                     self.push_rewrite(item.span, rewrite);
                 }
-                ast::ItemKind::Delegation(..) => {
+                ast::ItemKind::Delegation(..) | ast::ItemKind::DelegationMac(..) => {
                     // TODO: rewrite delegation items once syntax is established.
                     // For now, leave the contents of the Span unformatted.
                     self.push_rewrite(item.span, None)