serializer: force serialization as array

[baloo]

The implementation of array deserializer expects the payload to be serialized as array. Sadly the serializer left the door open for the underlying serializer to choice either bytes or array, if available.

This would only occur when the objects are used outside yubihsm.rs.

This change was tested on both mockhsm and yubihsm on usb.

[tony-iqlusion]

What concrete problem is the current implementation causing? It's an issue when using these serializers with some other serialization format?

[baloo]

I'm reusing the yubihsm::authentication::key::Key object for a project, and I'm sending it over the wire.
Sadly the Deserialize implementation (located further down) expects an array, while the Serialize doesn't specify the format.

Because we don't specify the serialization (whether we want array or bytes), this leaves the choice open to the serializer implementation used. This works out here because our serializer implementation defaults to array, but the other serializer I'm using (postcard) uses another default (I believe it uses bytes)

This change keeps both implementations of serde::Serialize and serde::Deserialize consistent.