RawBlock: switch to Cell or AtomicPtr instead of UnsafeCell

RawBlock: switch to Cell or AtomicPtr instead of UnsafeCell

This is the first step towards enabling multithreading on AtomTable.

For now RawBlock will default to using Cell, which yields a byte-
equivalent compiled output.

Also adds an `atomic` feature, which, when enabled, will make RawBlock
use an AtomicPtr instead, ensuring that it implements `Sync`.