Make /tools/improve script measure speed 5 times and compute an average

I measured speed 5 times manually too before introducing the script. It
gives more stable results.

tools/impact

@@ -12,10 +12,17 @@ prepare() {
   jake build
 }
 
-measure_speed() {
+run_benchmark() {
   echo $(jake benchmark | awk 'BEGIN { FS = " *│ *" } /Total/ { split($5, a, " "); print a[1] }')
 }
 
+measure_speed() {
+  bc <<-EOT
+    scale = 2
+    ($(run_benchmark) + $(run_benchmark) + $(run_benchmark) + $(run_benchmark) + $(run_benchmark)) / 5
+EOT
+}
+
 measure_size() {
   for file in examples/*.pegjs; do
     bin/pegjs "$file"