parser support anon iterators

tests/parser/tinvwhen.nim

@@ -0,0 +1,15 @@
+discard """
+  file: "tinvwhen.nim"
+  line: 11
+  errormsg: "invalid indentation"
+"""
+# This was parsed even though it should not!
+
+proc chdir(path: cstring): cint {.importc: "chdir", header: "dirHeader".}
+
+proc getcwd(buf: cstring, buflen: cint): cstring
+    when defined(unix): {.importc: "getcwd", header: "<unistd.h>".} #ERROR_MSG invalid indentation
+    elif defined(windows): {.importc: "getcwd", header: "<direct.h>"}
+    else: {.error: "os library not ported to your OS. Please help!".}
+
+