Apply impl lifetime elision rules

[theJian]

rust 2018 edition adds an additional elision rule for impl header.

related: #1273

[theJian]

This chapter doesn't cover the '_ shorthand. What do you think about adding some details on it?

[carols10cents]

Thanks! Unfortunately, the anonymous lifetime definitely needs to be introduced before we use it here. Additionally, the text after the first place you've changed needs to be updated, and the whole section should be checked to make sure it still makes sense.

So I'm going to close this in favor of a larger effort to incorporate the anonymous lifetime, which is tracked by #1273. Thanks though!