Guide testing

The start of a testing guide. This PR relies on the crates and modules one because I shuffled some stuff around, so sorry about that.

I got stuck with how to import this name with cargo. @wycats and @alexcrichton , any ideas?
Not sure if you want to mention in detail here, but AFAIK it's considered better style to do as below for Functions, but to do the above for Types & Traits?