Implement h_platform_stopwatch for Windows

We use QueryPerformanceCounter which will return realtime, not user
time.

src/platform_win32.c

@@ -8,3 +8,15 @@ void h_platform_errx(int err, const char* format, ...) {
   ExitProcess(err);
 }
 
+void h_platform_stopwatch_reset(struct HStopWatch* stopwatch) {
+  QueryPerformanceFrequency(&stopwatch->qpf);
+  QueryPerformanceCounter(&stopwatch->start);
+}
+
+/* return difference between last reset point and now */
+int64_t h_platform_stopwatch_ns(struct HStopWatch* stopwatch) {
+  LARGE_INTEGER now;
+  QueryPerformanceCounter(&now);
+
+  return 1000000000 * (now.QuadPart - stopwatch->start.QuadPart) / stopwatch->qpf.QuadPart;
+}

tools/windows/hammer_lib_src_list

 platform_win32.c 
-allocator.c 
+allocator.c
+benchmark.c
 bitreader.c 
 bitwriter.c 
 cfgrammar.c