Make Range a native object type.

Still need to implement better semantics, but this is an 
important first step. It's much faster now too, which is good.

test/range/iterate.wren

@@ -0,0 +1,8 @@
+var range = 1..3
+IO.print(range.iterate(null)) // expect: 1
+IO.print(range.iterate(1)) // expect: 2
+IO.print(range.iterate(2)) // expect: 3
+IO.print(range.iterate(3)) // expect: false
+IO.print(range.iterate(4)) // expect: false
+
+// TODO: Negative and empty ranges.