[Standard library] Add a bunch of explicit typealiases for associated types.

Type witness inference is failing in a large number of places due to the
adoption of conditional conformances in Slice. Add the necessary typealiases
to get the standard library to type-check.

This is a hack that can be reverted once type witness inference is improved.

Author: DougGregor

stdlib/public/core/CocoaArray.swift

@@ -29,6 +29,8 @@ import SwiftShims
 @_fixed_layout
 internal struct _CocoaArrayWrapper : RandomAccessCollection {
   typealias Indices = CountableRange<Int>
+  typealias SubSequence = Slice<_CocoaArrayWrapper>
+  
   @_inlineable
   @_versioned
   internal var startIndex: Int {

stdlib/public/core/DoubleWidth.swift.gyb

@@ -177,6 +177,8 @@ public struct DoubleWidth<Base : FixedWidthInteger> :
 
   @_fixed_layout // FIXME(sil-serialize-all)
   public struct Words : Collection {
+    public typealias Element = UInt
+
     public enum _IndexValue {
       case low(Base.Magnitude.Words.Index)
       case high(Base.Words.Index)

stdlib/public/core/Mirror.swift

@@ -564,7 +564,8 @@ internal extension Mirror {
   @_versioned // FIXME(sil-serialize-all)
   internal struct LegacyChildren : RandomAccessCollection {
     internal typealias Indices = CountableRange<Int>
-    
+    internal typealias SubSequence = Slice<LegacyChildren>
+
     @_inlineable // FIXME(sil-serialize-all)
     @_versioned // FIXME(sil-serialize-all)
     internal init(_ oldMirror: _Mirror) {
@@ -870,6 +871,7 @@ public struct DictionaryLiteral<Key, Value> : ExpressibleByDictionaryLiteral {
 /// interoperate with the rest of the standard library.
 extension DictionaryLiteral : RandomAccessCollection {
   public typealias Indices = CountableRange<Int>
+  public typealias SubSequence = Slice<DictionaryLiteral>
 
   /// The position of the first element in a nonempty collection.
   ///

stdlib/public/core/StringCharacterView.swift

@@ -194,7 +194,8 @@ extension String.CharacterView : BidirectionalCollection {
   internal var unicodeScalars: UnicodeScalarView {
     return UnicodeScalarView(_core, coreOffset: _coreOffset)
   }
-  
+
+  public typealias Element = Character
   public typealias Index = String.Index
   public typealias IndexDistance = Int
 

stdlib/public/core/StringCore.swift

@@ -662,7 +662,8 @@ public struct _StringCore {
 extension _StringCore : RandomAccessCollection {
 
   public typealias Indices = CountableRange<Int>
-
+  public typealias SubSequence = _StringCore
+  
   @_inlineable // FIXME(sil-serialize-all)
   public // @testable
   var startIndex: Int {

stdlib/public/core/StringRangeReplaceableCollection.swift.gyb

@@ -10,7 +10,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-extension String : StringProtocol, RangeReplaceableCollection {  
+extension String : StringProtocol, RangeReplaceableCollection {
+  public typealias Element = Character
+  
   /// A type that represents the number of steps between two `String.Index`
   /// values, where one value is reachable from the other.
   ///

stdlib/public/core/StringUTF16.swift

@@ -104,6 +104,7 @@ extension String {
     CustomStringConvertible,
     CustomDebugStringConvertible {
 
+    public typealias Element = UTF16.CodeUnit
     public typealias Index = String.Index
     public typealias IndexDistance = Int
 
@@ -447,6 +448,7 @@ extension String.UTF16View : CustomPlaygroundQuickLookable {
 }
 
 extension String.UTF16View.Indices : BidirectionalCollection {
+  public typealias Element = String.UTF16View.Index
   public typealias Index = String.UTF16View.Index
   public typealias IndexDistance = String.UTF16View.IndexDistance
   public typealias Indices = String.UTF16View.Indices

stdlib/public/core/StringUTF8.swift

@@ -119,6 +119,7 @@ extension String {
       self._legacyOffsets = (Int8(legacyOffsets.0), Int8(legacyOffsets.1))
     }
 
+    public typealias Element = UTF8.CodeUnit
     public typealias Index = String.Index
     public typealias IndexDistance = Int
 

stdlib/public/core/StringUnicodeScalarView.swift

@@ -95,6 +95,7 @@ extension String {
       }
     }
 
+    public typealias Element = UnicodeScalar
     public typealias Index = String.Index
     public typealias IndexDistance = Int
 

stdlib/public/core/Substring.swift.gyb

@@ -439,6 +439,7 @@ extension Substring {
 }
 
 extension Substring.${View} : BidirectionalCollection {
+  public typealias Element = String.${View}.Element
   public typealias Index = String.Index
   public typealias SubSequence = Substring.${View}
 

stdlib/public/core/UnicodeScalar.swift

@@ -418,7 +418,7 @@ extension Unicode.Scalar {
 }
 
 extension Unicode.Scalar.UTF16View : RandomAccessCollection {
-
+  public typealias SubSequence = Slice<Unicode.Scalar.UTF16View>
   public typealias Indices = CountableRange<Int>
 
   /// The position of the first code unit.