curve: use derive macro for zeroizing

[AaronFeickert]

Several types implement Zeroize manually. It seems safer to do this via the supplied derive macro, since any field changes will be caught automatically.

This PR moves to this functionality.

[tarcieri]

Note: the proc macro stack is not currently a hard dependency of curve25519-dalek (this is where having Cargo.lock checked in would be useful for reviewing the effects of dependency changes, particularly in conjunction with GitHub bots that can help monitor / analyze them)

Enabling this feature, while it would save 50 lines and automatically catch field additions / removals, would also pull in the following additional dependencies which are not currently required:

• proc-macro2
• unicode-ident
• quote
• syn
• zeroize_derive

Now granted, these are commonly used dependencies, but it is a pretty big increase in total dependency surface.

Regarding the problem of field additions/changes: I don't think that's a particularly big problem in practice with this set of types, which are more or less all in their "final form".

[AaronFeickert]

That's a fair point. Happy to close if you don't think the added baggage is worth it in this case. (Or a maintainer can close if they wish.)

[tarcieri]

I think the extra dependencies probably aren't worth the ~50loc it saves in this case, although that could change in the future if there are more usages of zeroize.

[AaronFeickert]

Noted! Closing.