zeroize: potetial redesign ideas

Empieza revisando la API propuesta en este issue y el debate sobre la zeroización plana en #1045. Aclara si el proyecto quiere un rediseño, una adición compatible con versiones anteriores o un cambio para v2.0; se considera terminado cuando se hayan acordado la dirección de la API y el alcance de la migración.

Right now I see the following issues with public API of zeroize:

• Derived and manually implemented Zeroize can be inefficient since they rely on zeroization of fields one by one or on zeroizing Drop impls.
• Procedural macros are relatively heavy compile time-wise, so usually we do not derive Zeroize ad rely on manual implementations.
• ZeroizeOnDrop is a somewhat useless trait, I haven't seen it used in practice.
• Zeroizing usefulness is limited. It can be used only for "primitive" types (e.g. raw keys), complex types do not implement Zeroize and instead implement zeroizing Drop.
• Zeroization is enabled for a whole crate using features. It's not possible to use selective zeroization and zeroization is not reflected in any way in user code.

I think most structures should be zeroized using the "flat" zeroization (see #1045) and that it can be useful to have explicit indication in user code of structs being zeroized on drop.

So I would like to suggest roughly this API:

/// Zeroizes memory pointed by `data_ptr`, but not memory
/// potentially reference by `T`.
pub unsafe fn zeroize_flat<T>(data_ptr: *mut T) {
    // ...
}

/// Zeroize-on-drop wrapper.
///
/// Note that this wrapper zeroizes only data owned by `T`
/// and does nothing with data referenced by it.
#[repr(transparent)]
pub struct ZeroizeOnDrop<T, const IS_ENABLED: bool = true>(T);

/// Abbreviated alias for `ZeroizeOnDrop`. 
pub type Zod<T, const IS_ENABLED: bool = true> = ZeroizeOnDrop<T, IS_ENABLED>;

impl<T, const IS_ENABLED: bool> Drop for ZeroizeOnDrop<T, IS_ENABLED> {
    fn drop (&mut self) {
        unsafe {
            std::ptr::drop_in_place(&mut self.0);
            if IS_ENABLED { zeroize_flat(&mut self.0); }
        }
        
    }
}

// Impl Deref and DerefMut for `ZeroizeOnDrop`

UPD: FlatPod and FlatZod are removed.

It can be introduced in a backward-compatible way, but for clarity it's probably worth to release it as v2.0.

Users would write code like this:

pub struct Foo {
    secret_cipher: Zod<Aes128>,
    secret_key: Box<Zod<[u8; 16]>>,
    non_secret_hasher: Sha256,
    // other fields
}

const ZOD: bool = cfg!(feature = "zeroize");

pub struct Bar1 {
    // This field will be zeroized only if `zeroize` feature is enabled
    cipher: Zod<Aes128, ZOD>,
}

// Alternatively:
pub struct Bar2<const ZOD: bool = true> {
    cipher: Zod<Aes128, ZOD>,
}

While it will be a bit less convinient, I think it's useful to have explicit indication in source code that secret types will be zeroized on drop. We may provide aliases like type ZodAes128 = Zod<Aes128> to improve visibility, but I don't think they are worth the trouble and it should be sufficient to simply references zeroize in docs.