SharedPointer<T, P>: {Send,Sync} should require T: Send + Sync

[yvt]

The following program should fail to compile, but it doesn't and crashes at runtime:

use std::cell::Cell;

type Arc<T> = archery::SharedPointer<T, archery::ArcK>;

fn main() {
    let cell = Arc::new(Cell::new(None));

    let jhs = [(); 2].map(|()| {
        let cell = Arc::clone(&cell);
        std::thread::spawn(move || loop {
            // concurrent access of a `!Sync` cell from multiple threads
            cell.take();
            cell.set(Some(Box::new(42)));
        })
    });

    for jh in jhs {
        jh.join().unwrap();
    }
}

$ cargo run --release --quiet
free(): unaligned chunk detected in tcache 2
fish: Job 1, 'cargo run --release' terminated by signal SIGABRT (Abort)

[orium]

Wow. Thanks for finding and fixing this. Merged.