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.
Of the 13% of functions which still require explicit lifetimes, do any seem particularly notable for their nonconformity to the usual patterns? It would also be really great if you could select one of these real-world functions and use it in the example error message above.