Get rid of ZeroizableByteArray

[fjarri]

Currently we have a ZeroizableByteArray which is a thin wrapper around GenericArray. GenericArray does not currently support zeroization (see the PR at fizyk20/generic-array#126), and we need to put it in a SecretBox to return from SerializableToSecretArray::to_secret_array(). Two ways to fix this issue:

• Switch to using Rust arrays. This is blocked by const generics not supporting all the required functionality yet (e.g. using an associated constant from a trait in a const generic trait method, or statically-typed array splitting).
• Use GenericArray when the PR above is merged.

Given how slow the development of generic-array goes recently, there's no way to tell which comes first.

[fjarri]

fizyk20/generic-array#126 was merged, so ZeroizableByteArray can be cut out of #97