update for generators

[steveklabnik]

This issue tracks inclusion of information about generators into the Book. This will happen in the 2018 edition.

Tracking issue: rust-lang/rust#43122

[adamchalmers]

Generators didn't make it into Rust 2018, should we remove the "Blocking Rust 2018" label?

[steveklabnik]

Absolutely!

[marioidival]

Hi, I'm closing this issue just to clean up the items that already exist. If you think this issue makes sense to stay open, please create a new issue with updated information and mention this one.

thanks!