Fix parsing for `#![inner]` attributes followed by `#[outer]` ones

[Bromeon]

The initial implementation from #28 cannot handle this:

impl MyTrait for MyStruct {
    #![inner]
    #[outer]
    pub type MyType = std::string::String;
}

This pull request fixes that, by first parsing all attributes of the form #![inner] and then #[outer].

The tests now also include such cases.