CoreUtil* structs to facilitate common behavior

[syrmel]

I'd like to propose adding CoreUtil* structs to common to make it easier to provide consistent behavior across utils.

For example, this would enable:

const app = common.CoreutilInfo{
	name: 'tac'
	description: 'concatenate and print files in reverse'
}
...
mut fp := app.make_flag_parser(os.args)
...
file = os.open(fname) or {
    app.quit(message: "${fname}: read error")
}
app.quit(message: "Success!", return_code: 0)
app.quit(message: "Not sure how we can be here.", return_code: 127, show_help_advice: true)

If accepted, I am happy to do some refactoring to use it in the existing utilities.

[JalonSolov]

That's exactly what common is meant to do... collect common code, so it doesn't have to be duplicated across the other routines.