Fixed UB and implemented the Drop trait for atomic-queue

[sidit77]

If you have a reference &T, then normally in Rust the compiler performs optimizations based on the knowledge that &T points to immutable data. Mutating that data, for example through an alias or by transmuting an &T into an &mut T, is considered undefined behavior. UnsafeCell opts-out of the immutability guarantee for &T: a shared reference &UnsafeCell may point to data that is being mutated. This is called “interior mutability”.

Source
crossbeam-queue for reference.

I added the required UnsafeCells and also manually implemented the Drop trait as MaybeUninit<T> prevents the Drop implementation of T from running. This leads to memory leaks when the queue is used with types like String that use Drop to free thier memory.

[yamadapc]

Thank you!

[yamadapc]

Follow-up for tests tracked by #609

[yamadapc]

Builds fail due to coveralls key missing on linux PR actions; which needs to be fixed.