Rethink zeroization when ZeroizeOnDrop is available

[fjarri]

We may want to change SecretBox trait bounds and remove CanBeZeroizedOnDrop in favor of zeroize::ZeroizeOnDrop, which is available in zeroize since in 1.5. See RustCrypto/utils#699 and RustCrypto/utils#652.

In particular, the approximate course of action is:

• Derive ZeroizeOnDrop for public secret types (SecretKey, Signer, SecretKeyFactory)
• Assert the third-party types we use internally implement ZeroizeOnDrop (e.g. SigningKey). This can be asserted in compile-time.
• We may be able to use the objects from the secrecy crate (e.g. SecretBox) instead of our custom one.
• Also see the static_assertions crate if a static check is needed that backend types implement ZeroizeOnDrop.

Currently blocked by chacha20poly1305 (at 0.9) conflicting with zeroize 1.5.