[WIP] Add the pin_project attribute

[taiki-e]

Add a new attribute macro. This works as follows:

#[pin_project(self)]
fn foo(mut self: Pin<&mut Self>) {
    let x: Pin<&mut i32> = self.field1;
    let y: &mut i32 = self.field2;
}

// desugaring
fn foo(self: Pin<&mut Self>) {
    let mut __self = self.project();
    let x: Pin<&mut i32> = __self.field1;
    let y: &mut i32 = __self.field2;
}

TODO: write docs

[taiki-e]

I don't plan to put this feature in the next version. In the next version, we will stick to making pin-project safer.