Use a union to avoid UB with uninitialized &mut T

[SimonSapin]

The soundness rules of unsafe code are not completely settled in the language, but in some proposals creating &mut T with an uninitialized T value is Undefined Behavior even if nothing reads from that reference.

In a union with a zero-size variant however, being uninitialized is a valid state since the storage for that variant is entirely padding.

Bumping the version number because of the new T: Copy bound on Uninitialized, which is require because unions with non-Copy fields rust-lang/rust#32836 (or the MaybeUninit union itself rust-lang/rust#53491) are not stable in the language yet.