25: Support custom Drop implementation r=taiki-e a=taiki-e

```rust
use pin_project_lite::pin_project;

pin_project! {
    pub struct Struct<'a> {
        was_dropped: &'a mut bool,
        #[pin]
        field: u8,
    }
    impl PinnedDrop for Struct<'_> {
        fn drop(this: Pin<&mut Self>) { // <----- NOTE: this is not `self`
            **this.project().was_dropped = true;
        }
    }
}

fn main() {
    let mut was_dropped = false;
    drop(Struct { was_dropped: &mut was_dropped, field: 42 });
    assert!(was_dropped);
}
```

It's clear how to pass options(=arguments in `#[pin_project]`), and we don't have to think about how to pass options.
So, this is easiest to implement compared to other options.
For all other options, we have to start by deciding how to pass the options, which can be a hard task, whether simple or complex to implement.


Co-authored-by: Taiki Endo <[email protected]>