stdlib: Move Generator and _Element typealiases to Sequence and Collection protocols

The requirement machine does not allow 'where' clauses to reference
typealiases defined in protocol extensions.

These should probably be removed entirely since they're not useful
anymore, but I'd rather that decision was made by the stdlib folks.

Author: slavapestov

stdlib/public/core/Collection.swift

@@ -344,6 +344,12 @@ public protocol Collection: Sequence {
   // FIXME: Associated type inference requires this.
   override associatedtype Element
 
+  // FIXME: <rdar://problem/34142121>
+  // This typealias should be removed as it predates the source compatibility
+  // guarantees of Swift 3, but it cannot due to a bug.
+  @available(swift, deprecated: 3.2, obsoleted: 5.0, renamed: "Element")
+  typealias _Element = Element
+
   /// A type that represents a position in the collection.
   ///
   /// Valid indices consist of the position of every element and a

stdlib/public/core/MigrationSupport.swift

@@ -319,15 +319,6 @@ extension Substring: _CustomPlaygroundQuickLookable {
 #endif
 
 extension Collection {
-  // FIXME: <rdar://problem/34142121>
-  // This typealias should be removed as it predates the source compatibility
-  // guarantees of Swift 3, but it cannot due to a bug.
-  @available(*, unavailable, renamed: "Iterator")
-  public typealias Generator = Iterator
-
-  @available(swift, deprecated: 3.2, obsoleted: 5.0, renamed: "Element")
-  public typealias _Element = Element
-
   @available(swift, deprecated: 4.0, obsoleted: 5.0, message: "all index distances are now of type Int")
   public func index<T: BinaryInteger>(_ i: Index, offsetBy n: T) -> Index {
     return index(i, offsetBy: Int(n))

stdlib/public/core/Sequence.swift

@@ -330,6 +330,12 @@ public protocol Sequence {
   /// encapsulates its iteration state.
   associatedtype Iterator: IteratorProtocol where Iterator.Element == Element
 
+  // FIXME: <rdar://problem/34142121>
+  // This typealias should be removed as it predates the source compatibility
+  // guarantees of Swift 3, but it cannot due to a bug.
+  @available(*, unavailable, renamed: "Iterator")
+  typealias Generator = Iterator
+
   /// A type that represents a subsequence of some of the sequence's elements.
   // associatedtype SubSequence: Sequence = AnySequence<Element>
   //   where Element == SubSequence.Element,