next steps for closure iterators

tests/iter/tanoniter1.nim

@@ -0,0 +1,32 @@
+discard """
+  output: '''1
+2
+3
+4
+1
+2'''
+"""
+
+proc factory(a, b: int): iterator (): int =
+  iterator foo(): int =
+    var x = a
+    while x <= b:
+      yield x
+      inc x
+  return foo
+
+proc factory2(a, b: int): iterator (): int =
+  return iterator (): int =
+    var x = a
+    while x <= b:
+      yield x
+      inc x
+
+let foo = factory 1, 4
+
+for f in foo():
+  echo f
+
+let foo2 = factory2 1,2
+
+for f in foo2(): echo f