Default and positional arguments

Add defaults and positional arguments for function/method/closure declaration/call in Rust.
This RFC proposal is written from the ideas and suggestions brought in the corresponding feature issue in Rust main repository https://github.com/rust-lang/rust/issues/6973

[Rendered view](https://github.com/KokaKiwi/rfcs/blob/default_args/active/0000-default-arguments.md)

I do not see how this helps.

If I remember correctly, the main issue raised was the absence of overloading in the presence of a variety of parameters. `fillRectangle` has 12 overloads, for example (it should have 16 to be consistent), and even trimming it down to the bare essential it retains 2:

    fillRect(QRect const&, QBrush const&)
    fillRect(QRectF const&, QBrush const&)

Whether you have default or positional arguments does not help in generating bindings here.

And in general, you can generate bindings by simply "forgetting" default arguments. Certainly the resulting interface is less friendly (you now have to specify all parameters on each call), but it is automatically generated with no pain.

Thus, you might actually be undermining what is otherwise an interesting proposal. I would simply consider dropping that motivation.