Refine bench_record and bench_playback:

tools/Stats.h

+#ifndef Stats_DEFINED
+#define Stats_DEFINED
+
+struct Stats {
+    Stats(const double samples[], int n) {
+        min = samples[0];
+        max = samples[0];
+        for (int i = 0; i < n; i++) {
+            if (samples[i] < min) { min = samples[i]; }
+            if (samples[i] > max) { max = samples[i]; }
+        }
+
+        double sum = 0.0;
+        for (int i = 0 ; i < n; i++) {
+            sum += samples[i];
+        }
+        mean = sum / n;
+
+        double err = 0.0;
+        for (int i = 0 ; i < n; i++) {
+            err += (samples[i] - mean) * (samples[i] - mean);
+        }
+        var = err / (n-1);
+    }
+
+    double min;
+    double max;
+    double mean;  // Estimate of population mean.
+    double var;   // Estimate of population variance.
+};
+
+#endif//Stats_DEFINED