The types involved are all simple 1-tuple newtypes where zeroization
only involves calling `zeroize` on the inner type, making all of the
involved impls relatively trivial.
Avoiding custom derive arguably improves auditability as you don't need
to expand a proc macro to see the resulting code. It decreases the
number of required dependencies in order for the `zeroize` feature to
work, where some of those dependencies are incredibly heavy
(particularly `syn`).
