RFC: new lifetime elision rules

[Rendered](https://github.com/aturon/rfcs/blob/lifetime-elision/active/0000-lifetime-elision.md)

Note: the core idea for this RFC and the initial survey both came from @wycats.
Ah and now I just saw @aturon 's comment [here](https://github.com/rust-lang/rfcs/pull/141#issuecomment-47379548) which explicitly confirms that the intent has been to *not* consider an impl header in scope for lifetime elision on methods.