Move trait bound of Hashable to trait definition (#117)
* Support no_encode for ssz_derive * ssz: move trait bound of Hashable to trait definition * Get derive to work with the new hashable * Use new hashable interface for beacon crate
* Support no_encode for ssz_derive * ssz: move trait bound of Hashable to trait definition * Get derive to work with the new hashable * Use new hashable interface for beacon crate