Bunch<T> unconditionally implements Send/Sync

Affected versions of this crate unconditionally implements Send/Sync for Bunch<T>.
This allows users to insert T: !Sync to Bunch<T>. It is possible to create a data race to a T: !Sync by invoking the Bunch::get() API (which returns &T) from multiple threads. It is also possible to send T: !Send to other threads by inserting T inside Bunch<T> and sending Bunch<T> to another thread, allowing to create a data race by inserting types like T = Rc<_>.

Such data races can lead to memory corruption.