Remove Serializable and Deserializable implementations from slices and vecs

[irakliyk]

This PR removes current implementations of Serializable and Deserilizable traits from slices and vectors. The motivation for this is that the way sterilization worked for these structs was somewhat confusing (slices and vectors were serialized without their lengths). Instead, we can use explicit calls to write_batch() and read_batch() methods to accomplish the same thing.

In the future, we can reimplement Serializable and Deserializable on slices and vecs, but this time we would serialize them together with the number of elements (for which we can use usize serialization implemented in #238).

[plafer]

Left a suggestion; otherwise LGTM as is