We've received a number of complaints about `zeroize` compatibility
(see RustCrypto/utils#723), which was exacerbated by #884 which bumped
the `zeroize` version within a minor version series.

I had hoped the cargo resolver would've been able to select an older
compatible version of zeroize in these cases, but I've gotten a lot of
reports that isn't happening.

This release reverts #884 in hopes of supporting a wider range of
`zeroize` versions for now. It's technically breaking in the event
anyone is actually depending on `ZeroizeOnDrop` marker traits on these
types, but it seems we've already broken things for current users and
that's the much higher impact issue.