Remove pin_project! macro and add #[pinned_drop] attribute

[taiki-e]

This removes pin_project! block and adds #[pinned_drop] attribute as proc-macro-attribute.
If you want a custom Drop implementation, you need to pass the PinnedDrop argument to pin_project attribute instead of defining items in pin_project! block.

In the previous implementation, #[pinned_drop] implemented Drop directly, but this PR changes it to implement this via a private unsafe trait method.

Also, this renames pin_projectable to pin_project.

cc #21, #26
Related: #18 (comment)

Examples

Before:

use std::fmt::Debug;
use pin_project::{pin_project, pin_projectable};
use std::pin::Pin;

pin_project! {
    #[pin_projectable]
    pub struct Foo<T: Debug, U: Debug> {
        #[pin] pinned_field: T,
        unpin_field: U
    }

    #[pinned_drop]
    fn my_drop_fn<T: Debug, U: Debug>(foo: Pin<&mut Foo<T, U>>) {
        let foo = foo.project();
        println!("Dropping pinned field: {:?}", foo.pinned_field);
        println!("Dropping unpin field: {:?}", foo.unpin_field);
    }
}

After:

use std::fmt::Debug;
use pin_project::{pin_project, pinned_drop};
use std::pin::Pin;

#[pin_project(PinnedDrop)]
pub struct Foo<T: Debug, U: Debug> {
    #[pin] pinned_field: T,
    unpin_field: U
}

#[pinned_drop]
fn my_drop_fn<T: Debug, U: Debug>(foo: Pin<&mut Foo<T, U>>) {
    let foo = foo.project();
    println!("Dropping pinned field: {:?}", foo.pinned_field);
    println!("Dropping unpin field: {:?}", foo.unpin_field);
}

TODO

• Update docs

[Aaron1011]

@taiki-e: Looks good to me!

[taiki-e]

bors r+

[bors]

Build succeeded

• taiki-e.pin-project