Refactor serenity crate for spec and ssz for easier derive (#98)

* Refactor slot generic

* Fix consts

* Revert "Refactor slot generic"

This reverts commit 39d0a539.

* [WIP] serenity tidy up

* Unify ssz and ssz-hash

* Remove unused ssz-hash and ssz-hash-derive crate

* Implement deposit handling

* Fix compile