Update a benchmark for new indexing model

Author: Dave Abrahams

benchmark/single-source/StaticArray.swift

@@ -44,6 +44,8 @@ struct A2X<T : StaticArrayProtocol> : StaticArrayProtocol {
 struct StaticArray<
   T : StaticArrayProtocol
 > : StaticArrayProtocol, RandomAccessCollection, MutableCollection {
+  typealias Indices = CountableRange<Int>
+  
   init(_ defaultValue : T.ElemTy) { values = T(defaultValue) }
   var values : T
   func get(_ idx: Int) -> T.ElemTy { return values.get(idx) }