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.
Trait definitions themselves are also a form that offers lifetime positions.  That may or may not be relevant (I'll be posting a question about that soon), but should probably be addressed explicitly.