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.
Please also add examples of methods within `impl`s where the implementing trait and/or type has lifetime parameters itself, just to underline the scenarios I brought up in [my comment here](https://github.com/rust-lang/rfcs/pull/141#discussion_r14934954)