Allow naming the projected types

[taiki-e]

By passing an argument with the same name as the method to the attribute, you can name the projection type returned from the method:

#[pin_project(
    Replace,
    project = StructProj,
    project_ref = StructProjRef,
    project_replace = StructProjOwn,
)]
struct Struct<T> {
    #[pin]
    field: T,
}

let mut x = Struct { field: 0 };
let StructProj { field } = Pin::new(&mut x).project();
let StructProjRef { field } = Pin::new(&x).project_ref();
let StructProjOwn { field } = Pin::new(&mut x).project_replace(Struct { field: 1 });

cc #43
Closes #124

TODO:

• add tests
• write docs
• separate unrelated changes; done in Various cleanup #214
• msrv (unrestricted_attribute_tokens requires 1.34); done in Implement !Unpin option #219

[taiki-e]

bors r+

[bors]

Build succeeded:

• ci