move arg_type_at_bit to cargseq.h, and add tests for it and pass.
adjust code layout
Msg.h, make too long oneline throw excpetion expanded to multiple lines
refactor "added-args_ == ap_.count_args()" to added_all_args()
refactor, extract selecting next nonescape arg job from each add of Msg to next_nonescape_arg of ArgParser
refactor, extract arg type checking to validate_arg_type function