This is a mechanism to access non-Sync values in a Sync way.
When the user creates a &Inert<T> value from a &T, they must swear on the
holy baguette that they won't use the T directly until all the Inert
wrappers go away, while the various implementations of the NeutralizeUnsafe
trait make sure that the non-Sync behaviour of the T cannot be observed
through the wrapper.
Improve documentation, test the crate, make use of it.