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.
The rationale is that in several usage surveys, this was essentially the only pattern we saw when `&self` was involved.

I believe that the reason for this is that when you're borrowing something out of `self`, it makes sense to involve another ref for computation. In contrast, it's a very unusual pattern to borrow something out of a parameter as a method of some other object. It's just not really how people think about using methods and objects in general, so it doesn't happen (almost at all).

I suspect that in cases where this pattern **could** occur, people use standalone functions instead of methods.