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)

As overloading is not the purpose of this RFC, I think this motivation is "correct" as it help fixing some issues in FFI design with C++ code (not all, but at least the "default arguments" one)