Add a benchmark for wrenCall().

test/benchmark/api_call.wren

@@ -0,0 +1,9 @@
+class Benchmark {
+  foreign static call(iterations)
+}
+
+var result = Benchmark.call(1000000)
+// Returns false if it didn't calculate the right value. Otherwise returns the
+// elapsed time.
+System.print(result is Num)
+System.print("elapsed: %(result)")