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)

This is a bit strong, as only _stable_ libraries would have issues with changing, and not all of the standard library will be stable. I'm also not sure that they can't be changed in a backwards compatible way.