message: to.basic_hdl_sim_model: use None for signed if not signed or unsigned
diff: from hdlConvertor.hdlAst import HdlName, HdlBuiltinFn, HdlIntValue, HdlCall, iHdlExpr
  from hdlConvertor.translate.common.name_scope import LanguageKeyword
  +NONE = HdlName("None")
  +
  def BitsT(width, is_signed=False, bits_cls_name="Bits3t"):
  """
  @@ -19,7 +21,7 @@ def BitsT(width, is_signed=False, bits_cls_name="Bits3t"):
  c = HdlCall(HdlBuiltinFn.CALL, [
  HdlName(bits_cls_name, obj=LanguageKeyword()),
  HdlIntValue(width, None, None),
  - HdlIntValue(int(is_signed), None, None)
  + NONE if is_signed is None else HdlIntValue(int(is_signed), None, None)
  ])
  return c
  