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.
I think that it should use the lifetime of the *first* input parameter, regardless of whether it is self or not, and only if it is an elided lifetime.

This avoids issues with UFC and makes method and non-method functions work the same.

Supporting elision of lifetimes only in the return value when they are explicit on self seems a bad idea, since it is counterintuitive.