Use impl header lifetime elision in generated impls for '&' and '&mut' once stable

[LukasKalbertodt]

Just a reminder: once lifetime elision in impl headers (see tracking issue) is stable, the generated code should use that feature. So instead of:

impl<'a, T> Foo for &'a T {}

It will be (at least I think that's the syntax):

impl<T> Foo for &T {}

Same for &mut.

This does matter as the generated code ends up in the docs. And we want that code to be as idiomatic as possible.

[scottmcm]

Yes, that's correct. There's a bunch of examples from libcore in PR rust-lang/rust#54687.

[LukasKalbertodt]

@scottmcm Thanks for creating the stabilization for 2015 PR <3
Then, I can finally implement this issue.

[LukasKalbertodt]

As discussed in #44 implementing this is probably not really worth it. I will close this issue for now. If someone thinks this issue is absolutely worth solving, we can reopen.