[stdlib] indexing model: Interval/Range merge

This is step 1; we still need to introduce ClosedRange.

Author: Dave Abrahams

benchmark/single-source/PopFrontGeneric.swift

@@ -24,11 +24,11 @@ func _arrayReplace<B: _ArrayBufferProtocol, C: Collection
   target: inout B, _ subRange: Range<Int>, _ newValues: C
 ) {
   _precondition(
-    subRange.startIndex >= 0,
+    subRange.lowerBound >= 0,
     "Array replace: subRange start is negative")
 
   _precondition(
-    subRange.endIndex <= target.endIndex,
+    subRange.upperBound <= target.endIndex,
     "Array replace: subRange extends past the end")
 
   let oldCount = target.count

stdlib/private/StdlibCollectionUnittest/CheckCollectionType.swift

@@ -23,8 +23,8 @@ public struct SubscriptRangeTest {
 
   public func boundsIn<C : Collection>(c: C) -> Range<C.Index> {
     let i = c.startIndex
-    return c.advance(i, by: numericCast(bounds.startIndex)) ..<
-           c.advance(i, by: numericCast(bounds.endIndex))
+    return c.advance(i, by: numericCast(bounds.lowerBound)) ..<
+           c.advance(i, by: numericCast(bounds.upperBound))
   }
 
   public init(