86: Change #[pinned_drop] to trait implementation r=taiki-e a=taiki-e

proc-macro just rewrites the trait path and adds `unsafe`.
It needs to write the documentation more.

### Examples
Before:
```rust
use pin_project::{pin_project, pinned_drop, UnsafeUnpin};
use std::pin::Pin;

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

#[pinned_drop]
fn do_drop<T, U>(this: Pin<&mut Foo<T, U>>) {
    // ..
}

unsafe impl<T: Unpin, U> UnsafeUnpin for Foo<T, U> {}
```

After:

```rust
use pin_project::{pin_project, pinned_drop};
use std::pin::Pin;

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

#[pinned_drop]
impl<T, U> PinnedDrop for Foo<T, U> {
    fn drop(self: Pin<&mut Self>) {
        // ..
    }
}

unsafe impl<T: Unpin, U> UnsafeUnpin for Foo<T, U> {}
```

Closes #26

Co-authored-by: Taiki Endo <te316e89@gmail.com>

91: Make PinnedDrop safe trait r=taiki-e a=taiki-e

#86 (comment)

Co-authored-by: Taiki Endo <te316e89@gmail.com>

109: Release 0.4.0 r=taiki-e a=taiki-e

cc #21

### Changes since the latest 0.3 release:

* **Pin projection has become a safe operation.** In the absence of other unsafe code that you write, it is impossible to cause undefined behavior. (#18)

* `#[unsafe_project]` attribute has been replaced with `#[pin_project]` attribute. (#18, #33)

* The `Unpin` argument has been removed - an `Unpin` impl is now generated by default. (#18)

* Drop impls must be specified with `#[pinned_drop]` instead of via a normal `Drop` impl. (#18, #33, #86)

* `Unpin` impls must be specified with an impl of `UnsafeUnpin`, instead of implementing the normal `Unpin` trait. (#18)

* `#[pin_project]` attribute now determines the visibility of the projection type/method is based on the original type. (#96)

* `#[pin_project]` can now be used for public type with private field types. (#53)

* `#[pin_project]` can now interoperate with `#[cfg()]`. (#77)

* Added `project_ref` method to `#[pin_project]` types. (#93)

* Added `#[project_ref]` attribute. (#93)

* Removed "project_attr" feature and always enable `#[project]` attribute. (#94)

* `#[project]` attribute can now be used for `impl` blocks. (#46)

* `#[project]` attribute can now be used for `use` statements. (#85)

* `#[project]` attribute now supports `match` expressions at the position of the initializer expression of `let` expressions. (#51)

### Changes since the 0.4.0-beta.1 release:

* Fixed an issue that caused an error when using `#[pin_project(UnsafeUnpin)]` and not providing a manual `UnsafeUnpin` implementation on a type with no generics or lifetime. (#107)


Co-authored-by: Taiki Endo <te316e89@gmail.com>