Here we won't be adding any new features but rather we will be refactoring our code a touch. The mechanism we are going to use is modules.
Modules in Rust can be files or they can be inline, in this case we will use the
former making a model.rs file to contain our PullRequest struct. Following
the chapter of the book on modules should hopefully provide the basic syntax.
Remember to change your references to the PullRequest struct if you had any!
Through doing this we will also encounter visibility modifiers. By default all
visibility is private to the module unless explicitly stated via the pub keyword.