Provide way to import projection trait

[taiki-e]

The current project/project_into methods are trait-methods, so it needs to use glob import to use it from other modules.

We could probably provide the following way:

#[pin_project]
struct Foo<T> {
    #[pin]
    field: T,
}
mod bar {
    #[project]
    use super::Foo;
    // convert to
    use super::__FooProjectionTrait;
}

[taiki-e]

Blocked on #21 (comment)

[withoutboats]

Didn't search very hard but: why do you need the trait? I thought we made pin #[fundamental]?

edit: sorry, fundamental isn't relevant here.. but shouldnt they be valid inherent methods?

edit2: found #89, understand what happened now.

[taiki-e]

Closing this in favor of #89.