Add benchmark for string map keys.

benchmark/map_numeric.wren

@@ -0,0 +1,19 @@
+var start = IO.clock
+
+var map = {}
+
+for (i in 1..1000000) {
+  map[i] = i
+}
+
+var sum = 0
+for (i in 1..1000000) {
+  sum = sum + map[i]
+}
+IO.print(sum)
+
+for (i in 1..1000000) {
+  map.remove(i)
+}
+
+IO.print("elapsed: ", IO.clock - start)