[stdlib] Remove _buffer requirement from ArrayType

Until we get an optimizer pass to remove get/set pairs, passing a
property that is a protocol requirement as inout from generic code is
always going to cause an extra retain, causing many unintended Array
copies.

Because this dropped reference counts to 1 in some cases, it exercised
previously-untested code paths and uncovered bugs, particularly in the
handling of subrange replacement on Slice<T>.

There are still differences in speed for short arrays of CGPoint that bear
investigation, but at least as things scale up, the ratio of time goes
to 1.

Fixes <rdar://problem/17040913> append and += on an array have
completely different performance

Swift SVN r19228

stdlib/core/ContiguousArrayBuffer.swift

@@ -134,6 +134,18 @@ let emptyNSSwiftArray : _NSSwiftArray
     return self
   }
   
+  /// Replace the given subRange with the first newCount elements of
+  /// the given collection.
+  ///
+  /// Requires: this buffer is backed by a uniquely-referenced
+  /// ContiguousArrayBuffer
+  @public
+  mutating func replace<C: Collection where C.GeneratorType.Element == Element>(
+    #subRange: Range<Int>, with newCount: Int, elementsOf newValues: C
+  ) {
+    _arrayNonSliceInPlaceReplace(&self, subRange, newCount, newValues)
+  }
+  
   /// Get/set the value of the ith element
   @public subscript(i: Int) -> T {
     get {