Add required hidden definitions to some code examples

[Manishearth]

Otherwise the code examples fail to compile for the wrong reason.

This still doesn't compile because mdbook is forcing the 2015 edition, but that needs to be fixed as an mdbook config which I'm adding shortly.

[Manishearth]

Once rust-lang/mdBook#1093 re-reverted we can add the edition key

[cramertj]

Thanks!