Refactor enum ShredCode and enum ShredData

[alexpyattaev]

Problem

Since legacy shred were removed, enums ShredCode and ShredData have only a single variant. This is making API more convoluted than it needs to be.

Proposed Solution

Remove the indirections and inline merkle::ShredData and merkle::ShredCode as appropriate into enum Shred

This probably can be done separately for Data and Code shreds, this will help keep PRs smaller.

[AvhiMaz]

on it ser 🫡