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.

corelib.wren

@@ -21,3 +21,28 @@ class List {
     return result
   }
 }
+
+class Range {
+  new(min, max) {
+    _min = min
+    _max = max
+  }
+
+  min { return _min }
+  max { return _max }
+
+  iterate(previous) {
+    if (previous == null) return _min
+    if (previous == _max) return false
+    return previous + 1
+  }
+
+  iteratorValue(iterator) {
+    return iterator
+  }
+}
+
+class Num {
+  .. other { return new Range(this, other) }
+  ... other { return new Range(this, other - 1) }
+}