Reliable argument handling

[Stebalien]

Currently, we allow users to mix optional, variadic, and required arguments. In order of preference:

1. make this impossible (see below)
2. check at start
3. check when calling the command

We currently do none of these. Instead, we have some weird logic in CheckArguments where we stop filling in optional arguments once we realize we'll need the rest to fill in required arguments.

---

Preferred solution:

type Command struct {
	Arguments Arguments
	// ...
}
    
type Arguments struct {
	Required []Argument
	Optional []Argument
	Variadic Argument
}

[Stebalien]

Oh, and guess what! We do this dance twice (IN A ROW!).

[keks]

What do you mean with mix arguments? I agree the logic around checking the arguments is messed up, but I don't get why you want to split them in an Arguments struct.

[Stebalien]

Because we can, e.g., put optional arguments before mandatory arguments and variadic arguments before either.

[keks]

I think I start to get it. I'll be working on this once cmds2 is in.