Add member forwarding from Entry to its item

Progress on #15

Sources/Lotsawa/Chart.swift

@@ -273,6 +273,7 @@ extension Chart {
   }
 
   /// A Leo or Earley item bundled with a single mainstem cause.
+  @dynamicMemberLookup
   public struct Entry: Comparable, Hashable {
     var item: ItemID
 
@@ -298,6 +299,10 @@ extension Chart {
     public static func < (lhs: Self, rhs: Self) -> Bool {
       (lhs.item.key, lhs.mainstemIndexStorage) < (rhs.item.key, rhs.mainstemIndexStorage)
     }
+
+    subscript<Target>(dynamicMember m: KeyPath<ItemID, Target>) -> Target {
+      item[keyPath: m]
+    }
   }
 
   private func isItemIndexStart(_ i: Entries.Index) -> Bool {