Range operators!

Added ".." and "..." infix operators. Num implements them to
return Range objects. Those in turn implement the iterator
protocol, so now numeric for loops are easy:

    for (i in 1..10) { ... }

I also cleaned up the Wren benchmarks to use this.

In the process, I discovered the method_call benchmark really
just showed loop peformance, so I unrolled the loops in all of
the languages to stress method calls.

benchmark/fib.wren

@@ -7,9 +7,7 @@ var fib = fn(n) {
 }
 
 var start = OS.clock
-var i = 0
-while (i < 5) {
+for (i in 1..5) {
   IO.write(fib.call(28))
-  i = i + 1
 }
 IO.write("elapsed: " + (OS.clock - start).toString)