* iterator refactoring

Author: adamnemecek

Sources/DoublyLinkedList.swift

@@ -167,11 +167,10 @@ public struct DoublyLinkedList<Element>: CustomStringConvertible, Sequence {
     public func makeIterator() -> DoublyLinkedList.Iterator {
         var it = head
         return AnyIterator {
-            guard let e = it?.element else {
-                return nil
-            }
+          defer {
             it = it?.next
-            return e
+          }
+          return it?.element
 		}
 	}
 

Sources/RedBlackTree.swift

@@ -158,15 +158,8 @@ public struct RedBlackTree<Key: Comparable, Value>: Probable, Collection, Custom
 	//	:returns:	RedBlackTree.Generator
 	//
 	public func makeIterator() -> RedBlackTree.Iterator {
-        var index = startIndex
-        return AnyIterator {
-            if index < self.endIndex {
-                let i: Int = index
-                index += 1
-                return self[i]
-            }
-            return nil
-        }
+        var i = indices.makeIterator()
+        return AnyIterator { i.next().map { self[$0] } }
 	}
 
 	/**

Sources/SortedDictionary.swift

@@ -120,14 +120,8 @@ public struct SortedDictionary<Key: Comparable, Value>: Probable, Collection, Eq
 	}
 
 	public func makeIterator() -> SortedDictionary.Iterator {
-		var index = startIndex
-    return AnyIterator {
-      guard index < self.endIndex else { return nil }
-      defer {
-        index += 1
-      }
-      return self[index]
-		}
+    var i = indices.makeIterator()
+    return AnyIterator { i.next().map { self[$0] } }
 	}
 
 	/**

Sources/SortedMultiDictionary.swift

@@ -120,15 +120,8 @@ public struct SortedMultiDictionary<Key: Comparable, Value>: Probable, Collectio
     }
 
     public func makeIterator() -> SortedMultiDictionary.Iterator {
-        var index = startIndex
-        return AnyIterator {
-            if index < self.endIndex {
-                let i: Int = index
-                index += 1
-                return self[i]
-            }
-            return nil
-        }
+        var i = indices.makeIterator()
+        return AnyIterator { i.next().map { self[$0] } }
     }
 
     /**

Sources/SortedMultiSet.swift

@@ -157,15 +157,8 @@ public struct SortedMultiSet<T: Comparable>: Probable, Collection, Equatable, Cu
 	//	:returns:	SortedMultiSet.Generator
 	//	
 	public func makeIterator() -> SortedMultiSet.Iterator {
-        var index = startIndex
-        return AnyIterator {
-            if index < self.endIndex {
-                let i = index
-                index += 1
-                return self[i]
-            }
-            return nil
-        }
+        var i = indices.makeIterator()
+        return AnyIterator { i.next().map { self[$0] } }
 	}
 
     /**

Sources/SortedSet.swift

@@ -137,10 +137,8 @@ public struct SortedSet<T: Comparable>: Probable, Collection, Equatable, CustomS
 	}
 
 	public func makeIterator() -> SortedSet.Iterator {
-        var i = indices.makeIterator()
-        return AnyIterator {
-            i.next().map { self[$0] }
-        }
+      var i = indices.makeIterator()
+      return AnyIterator { i.next().map { self[$0] } }
 	}
 
 	/**