Add zeroize support

[jac-cbi]

I opened this discussion against liboqs-rust #269, but since it seems to be unmaintained, I'll open it here.

From my analysis of the code in safe-oqs, safe-oqs-sys, and liboqs, it seems that sensitive buffers like PrivateKey and SharedSecret aren't being properly zeroed out after use.

I'm basing this assessment on the fact that the buffers are initialized in Rust code (see newtype_buffer!() in macro.rs, but without an implementation of Drop.

If my assessment is correct, should we derive Zeroize and implement Drop for sensitive buffers? Or, implement Drop and call the liboqs OQS_MEM_secure_free()? My gut says that since the buffers are created in Rust, we should use the Rust zeroize crate.

Edit: please '@' me, as GitHub isn't in my daily workflow