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.
@wycats, what proportion of the cited 87% would be lost if this rule were not accepted? I don't personally object to it, but I can see how it's a bit more flimsy than the others, and I would be willing to live without it if the statistics bore it out.