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.
Okay, here is a gist with my attempt to survey the space here: https://gist.github.com/pnkfelix/a4054e51400152c63714

It could well be that the intent is (and has always been) to *not* consider an impl header in scope for lifetime elision on methods.  But if so, this needs to be spelled out explicitly in the RFC itself.