Eliminate the _*Indexable protocols.

The various _*Indexable protocols only exist to work around the lack of
recursive protocol constraints. Eliminate all of the *_Indexable protocols,
collapsing their requirements into the corresponding Collection protocol
(e.g., _MutableIndexable —> Collection).

This introduces a number of extraneous requirements into the various
Collection protocols to work around bugs in associated type
inference. Specifically, to work around the lack of "global" inference
of associated type witnesses. These hacks were implicitly present in
the *Indexable protocols; I've made marked them as ABI FIXMEs here so
we can remove them when associated type inference improves.

Fixes rdar://problem/21935030 and a number of ABI FIXMEs in the library.

Author: DougGregor

stdlib/public/SDK/Dispatch/Data.swift

@@ -299,6 +299,7 @@ public struct DispatchData : RandomAccessCollection, _ObjectiveCBridgeable {
 }
 
 public struct DispatchDataIterator : IteratorProtocol, Sequence {
+  public typealias Element = UInt8
 
 	/// Create an iterator over the given DispatchData
 	public init(_data: DispatchData) {

stdlib/public/core/BidirectionalCollection.swift

@@ -17,28 +17,7 @@
 /// `BidirectionalCollection` protocol instead, because it has a more complete
 /// interface.
 @available(*, deprecated, message: "it will be removed in Swift 4.0.  Please use 'BidirectionalCollection' instead")
-public typealias BidirectionalIndexable = _BidirectionalIndexable
-public protocol _BidirectionalIndexable : _Indexable {
-  // FIXME(ABI)#22 (Recursive Protocol Constraints): there is no reason for this protocol
-  // to exist apart from missing compiler features that we emulate with it.
-  // rdar://problem/20531108
-  //
-  // This protocol is almost an implementation detail of the standard
-  // library.
-
-  /// Returns the position immediately before the given index.
-  ///
-  /// - Parameter i: A valid index of the collection. `i` must be greater than
-  ///   `startIndex`.
-  /// - Returns: The index value immediately before `i`.
-  func index(before i: Index) -> Index
-
-  /// Replaces the given index with its predecessor.
-  ///
-  /// - Parameter i: A valid index of the collection. `i` must be greater than
-  ///   `startIndex`.
-  func formIndex(before i: inout Index)
-}
+public typealias BidirectionalIndexable = BidirectionalCollection
 
 /// A collection that supports backward as well as forward traversal.
 ///
@@ -65,10 +44,14 @@ public protocol _BidirectionalIndexable : _Indexable {
 ///   `c.index(before: c.index(after: i)) == i`.
 /// - If `i > c.startIndex && i <= c.endIndex`
 ///   `c.index(after: c.index(before: i)) == i`.
-public protocol BidirectionalCollection : _BidirectionalIndexable, Collection 
+public protocol BidirectionalCollection : Collection 
 {
+  // FIXME(ABI): Associated type inference requires this.
+  associatedtype Element
+
+  // FIXME(ABI): Associated type inference requires this.
+  associatedtype Index
 
-// TODO: swift-3-indexing-model - replaces functionality in BidirectionalIndex
   /// Returns the position immediately before the given index.
   ///
   /// - Parameter i: A valid index of the collection. `i` must be greater than
@@ -84,7 +67,7 @@ public protocol BidirectionalCollection : _BidirectionalIndexable, Collection
 
   /// A sequence that can represent a contiguous subrange of the collection's
   /// elements.
-  associatedtype SubSequence: BidirectionalCollection
+  associatedtype SubSequence : BidirectionalCollection
     = BidirectionalSlice<Self>
 
   /// A type that represents the indices that are valid for subscripting the
@@ -147,10 +130,19 @@ public protocol BidirectionalCollection : _BidirectionalIndexable, Collection
   /// - Parameter bounds: A range of the collection's indices. The bounds of
   ///   the range must be valid indices of the collection.
   subscript(bounds: Range<Index>) -> SubSequence { get }
+
+  // FIXME(ABI): Associated type inference requires this.
+  subscript(position: Index) -> Element { get }
+
+  // FIXME(ABI): Associated type inference requires this.
+  var startIndex: Index { get }
+
+  // FIXME(ABI): Associated type inference requires this.
+  var endIndex: Index { get }
 }
 
 /// Default implementation for bidirectional collections.
-extension _BidirectionalIndexable {
+extension BidirectionalCollection {
 
   @_inlineable // FIXME(sil-serialize-all)
   @inline(__always)