Support #[derive(FromRequest)] on enums

[davidpdrsn]

This adds support for #[derive(FromRequest)] on enums. It requires using #[from_request(via(OtherExtractor))] such as

#[derive(FromRequest, Clone)]
#[from_request(via(Extension))]
enum Env {
    Production,
    Development,
    Test,
}

A future improvement of this could be supporting something like

#[derive(FromRequest)]
enum Role {
    Admin(Admin),
    User(User),
    NotSignedIn,
}

Which would first attempt to extract Admin, then User, and finally fallback to Role::NotSignedIn. I might tackle that in the future :)

Fixes #997