Better argument validation for some list methods.

test/list/iterate.wren

@@ -0,0 +1,7 @@
+var a = ["one", "two", "three", "four"]
+IO.write(a.iterate(null)) // expect: 0
+IO.write(a.iterate(0)) // expect: 1
+IO.write(a.iterate(1)) // expect: 2
+IO.write(a.iterate(2)) // expect: 3
+IO.write(a.iterate(3)) // expect: false
+IO.write(a.iterate(-1)) // expect: false