Fix #2. Finish implementing Range.

Covers error and edge cases and fully tested now.

test/range/iterate.wren

@@ -1,3 +1,4 @@
+// Inclusive.
 var range = 1..3
 IO.print(range.iterate(null)) // expect: 1
 IO.print(range.iterate(1)) // expect: 2
@@ -5,4 +6,30 @@ 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.
+// Exclusive
+range = 1...3
+IO.print(range.iterate(null)) // expect: 1
+IO.print(range.iterate(1)) // expect: 2
+IO.print(range.iterate(2)) // expect: false
+
+// Negative inclusive range.
+range = 3..1
+IO.print(range.iterate(null)) // expect: 3
+IO.print(range.iterate(3)) // expect: 2
+IO.print(range.iterate(2)) // expect: 1
+IO.print(range.iterate(1)) // expect: false
+
+// Negative exclusive range.
+range = 3...1
+IO.print(range.iterate(null)) // expect: 3
+IO.print(range.iterate(3)) // expect: 2
+IO.print(range.iterate(2)) // expect: false
+
+// Empty inclusive range.
+range = 1..1
+IO.print(range.iterate(null)) // expect: 1
+IO.print(range.iterate(1)) // expect: false
+
+// Empty exclusive range.
+range = 1...1
+IO.print(range.iterate(null)) // expect: false