delegation: Implement list delegation

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

Author: petrochenkov

src/items.rs

@@ -739,8 +739,8 @@ impl<'a> FmtVisitor<'a> {
  (_, 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 @@ impl<'b, 'a: 'b> FmtVisitor<'a> {
  );
  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)