Add ZeroizeOnDrop support for Strobe.

[codahale]

This adds a derived ZeroizeOnDrop implementation for Strobe, allowing for the state to be securely erased in memory once an instance is dropped.

[rozbb]

Hi, I apologize for the delay. This is great! Do you think it'd be possible to also impl Zeroize on OpFlags as well, just for the sake of completion (that way it doesn't have to be #[zeroize(skip)]'d)? The protocol state of a STROBE session might be considered just as private as the sponge state.

[codahale]

Good point. The derive macro doesn’t play nice with bitflags, so I added a concrete impl for Zeroize.

[rozbb]

Fantastic, thank you!

[codahale]

Thanks for writing and maintaining this library!