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.
@nick29581, that same argument can be made for type inference. Just like type inference, nothing is stopping you from being fully explicit with lifetimes if you deem it's better for readability.