[Standard library] Duplicate subscript requirement in BidirectionalCollection.

The BidirectionalCollection protocol has no requirements that mention
the associated type 'SubSequence', which means it cannot be easily
inferred. Duplicate the subscript requirement from Collection into
BidirectionalCollection to aid type witness inference.

Author: DougGregor

stdlib/public/core/BidirectionalCollection.swift

@@ -134,6 +134,29 @@ public protocol BidirectionalCollection
   ///     
   /// - Complexity: O(1)
   var last: Iterator.Element? { get }
+
+  /// Accesses a contiguous subrange of the collection's elements.
+  ///
+  /// The accessed slice uses the same indices for the same elements as the
+  /// original collection uses. Always use the slice's `startIndex` property
+  /// instead of assuming that its indices start at a particular value.
+  ///
+  /// This example demonstrates getting a slice of an array of strings, finding
+  /// the index of one of the strings in the slice, and then using that index
+  /// in the original array.
+  ///
+  ///     let streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
+  ///     let streetsSlice = streets[2 ..< streets.endIndex]
+  ///     print(streetsSlice)
+  ///     // Prints "["Channing", "Douglas", "Evarts"]"
+  ///
+  ///     let index = streetsSlice.index(of: "Evarts")    // 4
+  ///     print(streets[index!])
+  ///     // Prints "Evarts"
+  ///
+  /// - 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 }
 }
 
 /// Default implementation for bidirectional collections.