Benchmark: implement memory query for Windows

Implement support for querying the high water mark for the working set
for the current process.  This is accomplished with the PSAPI (Version
2) API from Windows.

Sources/SwiftDocC/Benchmark/Metrics/PeakMemory.swift

@@ -9,6 +9,9 @@
 */
 
 import Foundation
+#if os(Windows)
+import WinSDK
+#endif
 
 extension Benchmark {
     /// A peak memory footprint metric for the current process.
@@ -55,6 +58,16 @@ extension Benchmark {
 
             return Int64(peakMemory * 1024) // convert from KBytes to bytes
         }
+        #elseif os(Windows)
+        private static func peakMemory() -> Int64? {
+            var pmcStats = PROCESS_MEMORY_COUNTERS()
+            guard K32GetProcessMemoryInfo(
+                GetCurrentProcess(),
+                &pmcStats,
+                DWORD(MemoryLayout<PROCESS_MEMORY_COUNTERS>.size)
+            ) else { return nil }
+            return Int64(pmcStats.PeakWorkingSetSize)
+        }
         #endif
 
         public var result: MetricValue? {