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/for.wren

@@ -1,12 +1,6 @@
 var list = []
 
-{
-  var i = 0
-  while (i < 2000000) {
-    list.add(i)
-    i = i + 1
-  }
-}
+for (i in 0...2000000) list.add(i)
 
 var start = OS.clock
 var sum = 0