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 the word `for` is lacking from the second example. It’s not an obvious example of where the lifetimes are, either—it could be rewritten as the probably-fairly-nonsensical “`impl<'a, 'b> SomeTrait<'a> for Foo<'b>` has `'a` in [the] output position and `'b` in [the] input position”.

(As for the “the”, I think that should be there in all these cases.)