Remove Arguments class

I will soon add another stage in the option parsing “pipeline”, which
will involve a new class for which I can’t find a better name than
`Arguments`. That class will actually have behavior rather than just
being a wrapper around two lists, so I’m removing the current class in
favor of it.

With that new stage, most people shouldn’t need to use parseArguments()
directly, so the slightly uglier return type shouldn’t matter too much.