[benchmark] Extract setup: MapReduceClass(Small)

MapReduceClass had setup overhead fo 868 μs (7%).

Setup overhead of MapReduceClassShort was practically lost in the measurement noise from it’s artificially high base load, but it was there.

Extracting the decimal array initialization into `SetUpFunction` also takes out the cost of releasing the [NSDecimalNumber], which turns out to be about half of the measured runtime in the case of MapReduceClass benchmark. This significantly changes the reported runtimes (to about half), therfore the modified benchmarks get a new name with suffix `2`.

benchmark/single-source/MapReduce.swift

@@ -17,8 +17,10 @@ public let MapReduce = [
   BenchmarkInfo(name: "MapReduce", runFunction: run_MapReduce, tags: [.validation, .algorithm]),
   BenchmarkInfo(name: "MapReduceAnyCollection", runFunction: run_MapReduceAnyCollection, tags: [.validation, .algorithm]),
   BenchmarkInfo(name: "MapReduceAnyCollectionShort", runFunction: run_MapReduceAnyCollectionShort, tags: [.validation, .algorithm]),
-  BenchmarkInfo(name: "MapReduceClass", runFunction: run_MapReduceClass, tags: [.validation, .algorithm]),
-  BenchmarkInfo(name: "MapReduceClassShort", runFunction: run_MapReduceClassShort, tags: [.validation, .algorithm]),
+  BenchmarkInfo(name: "MapReduceClass2", runFunction: run_MapReduceClass, tags: [.validation, .algorithm],
+    setUpFunction: { decimals(1000) }, tearDownFunction: releaseDecimals),
+  BenchmarkInfo(name: "MapReduceClassShort2", runFunction: run_MapReduceClassShort, tags: [.validation, .algorithm],
+    setUpFunction: { decimals(10) }, tearDownFunction: releaseDecimals),
   BenchmarkInfo(name: "MapReduceLazyCollection", runFunction: run_MapReduceLazyCollection, tags: [.validation, .algorithm]),
   BenchmarkInfo(name: "MapReduceLazyCollectionShort", runFunction: run_MapReduceLazyCollectionShort, tags: [.validation, .algorithm]),
   BenchmarkInfo(name: "MapReduceLazySequence", runFunction: run_MapReduceLazySequence, tags: [.validation, .algorithm]),
@@ -28,6 +30,17 @@ public let MapReduce = [
   BenchmarkInfo(name: "MapReduceString", runFunction: run_MapReduceString, tags: [.validation, .algorithm, .String]),
 ]
 
+#if _runtime(_ObjC)
+var decimals : [NSDecimalNumber]!
+func decimals(_ n: Int) {
+  decimals = (0..<n).map { NSDecimalNumber(value: $0) }
+}
+func releaseDecimals() { decimals = nil }
+#else
+func decimals(_ n: Int) {}
+func releaseDecimals() {}
+#endif
+
 @inline(never)
 public func run_MapReduce(_ N: Int) {
   var numbers = [Int](0..<1000)
@@ -149,7 +162,7 @@ public func run_MapReduceShortString(_ N: Int) {
 @inline(never)
 public func run_MapReduceClass(_ N: Int) {
 #if _runtime(_ObjC)
-  let numbers = (0..<1000).map { NSDecimalNumber(value: $0) }
+  let numbers: [NSDecimalNumber] = decimals
 
   var c = 0
   for _ in 1...N*100 {
@@ -163,7 +176,7 @@ public func run_MapReduceClass(_ N: Int) {
 @inline(never)
 public func run_MapReduceClassShort(_ N: Int) {
 #if _runtime(_ObjC)
-  let numbers = (0..<10).map { NSDecimalNumber(value: $0) }
+  let numbers: [NSDecimalNumber] = decimals
 
   var c = 0
   for _ in 1...N*10000 {
@@ -173,4 +186,3 @@ public func run_MapReduceClassShort(_ N: Int) {
   CheckResults(c != 0)
 #endif
 }
-