Add zeroize

[aewag]

https://crates.io/crates/zeroize

EDIT:
Wait until new version is released as changes/extensions are on the way (i.e. RustCrypto/utils#651 RustCrypto/utils#652)'

[aewag]

No support for tinyvec

error[E0599]: the method `zeroize_or_on_drop` exists for mutable reference `&mut ArrayVec<[u8; 5]>`, but its trait bounds were not satisfied
   --> src/main.rs:6:46
    |
6   | #[derive(Debug, Default, PartialEq, Zeroize, ZeroizeOnDrop)]
    |                                              ^^^^^^^^^^^^^ method cannot be called on `&mut ArrayVec<[u8; 5]>` due to unsatisfied trait bounds
    |
   ::: /home/alwagner/.cargo/registry/src/github.com-1ecc6299db9ec823/tinyvec-1.5.1/src/arrayvec.rs:104:1
    |
104 | pub struct ArrayVec<A> {
    | ----------------------
    | |
    | doesn't satisfy `ArrayVec<[u8; 5]>: AssertZeroize`
    | doesn't satisfy `ArrayVec<[u8; 5]>: ZeroizeOnDrop`
    | doesn't satisfy `ArrayVec<[u8; 5]>: Zeroize`
    |
    = note: the following trait bounds were not satisfied:
            `ArrayVec<[u8; 5]>: Zeroize`
            which is required by `ArrayVec<[u8; 5]>: AssertZeroize`
            `&mut ArrayVec<[u8; 5]>: Zeroize`
            which is required by `&mut ArrayVec<[u8; 5]>: AssertZeroize`
            `ArrayVec<[u8; 5]>: ZeroizeOnDrop`
            which is required by `&mut ArrayVec<[u8; 5]>: AssertZeroizeOnDrop`
            `[u8]: Sized`
            which is required by `[u8]: AssertZeroize`
    = note: this error originates in the derive macro `ZeroizeOnDrop` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0599`.
error: could not compile `rustcrypto_utils_zeroize` due to previous error

EDIT: With RustCrypto/utils#719 zeroize can be used for ReferenceImplPrivateKey.