Add support for optional arguments to arg_check!

[rustpython]

Add support for optional arguments to arg_check!

[OddBloke]

This is more challenging than required arguments; as we don't necessarily have a value, we can't just set a variable in the caller. So there are a few options:

• convert all parsed arguments to Options
• only make optional arguments Options
• put optional arguments in to some sort of container (like a HashMap), that could also capture their optionality

The first option would make required arguments more verbose, without any real gain; so I'd prefer not to do that.

The other two options are both pretty good; using an Option feels like a better model, but might be harder to implement. In particular, specifying the macro pattern in a way that distinguishes between the Option and non-Option argument variables proved challenging when I was playing around with this locally. Even with the HashMap option, we would still have to work out how to specify the optional arguments in a good way.

A separate macro might make either of them much easier, but then the caller would be responsible for hooking up the macros (as the second would depend on how many arguments the first had consumed), which is best avoided, I think.

[cthulahoops]

Option 2 sounds best to me too.

At some point we have to deal with keyword arguments too, and the fact that the caller can pass positional arguments as keyword arguments.

[OddBloke]

Oh, oops, didn't mention this in the PR, I guess; I fixed this with #96 (using option 2!).