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)

@steveklabnik
> and not all of the standard library will be stable

Rust standard library API isn't supposed to be frozen post-1.0?
If so, that's what I mean when I say it can't be added post-1.0, as the API itself should be changed in order to use default args.
And add it post-1.0 could lead to keep "old-fashioned" functions in the standard library (I think)