0.2.2

ADDED

• You can now Pool::cast() a pool handle into a pool handle for a different type, allowing you to construct values of multiple types from the same pool, provided they are of the exact same size and alignment.
• Pools of size 0 are now represented by null pointers, meaning they allocate nothing. It also means Option<Pool> is no longer identical in size to Pool, but PoolRef still retains that property. A Pool of size 0 is also conceptually identical to the None value of an Option<Pool>, except you can use it to construct values without having to unwrap it first, so there's no good reason you should ever need Option<Pool>.