`'_` lifetime (RFC 2115)

[matthewjasper]

This is stabilized in 1.26.

• Add '_ to Lifetime grammar
• Explain how it works in lifetime-elision.md
• Use it in examples

[Havvy]

Is this complete? Or are we keeping open for if it becomes required in the 2018 edition as well?

[matthewjasper]

I've check for any other places where '_ could be used and I think that there aren't any.