'return' for first class iterators

tests/run/titer9.nim

@@ -0,0 +1,20 @@
+discard """
+  output: '''5
+14
+0'''
+"""
+
+iterator count(x: int, skip: bool): int {.closure.} =
+  if skip: return x+10
+  else: yield x+1
+
+  if skip: return x+10
+  else: yield x+2
+
+proc takeProc(x: iterator (x: int, skip: bool): int) =
+  echo x(4, false)
+  echo x(4, true)
+  echo x(4, false)
+
+takeProc(count)
+