versioning

[imabdulbasit]

This PR:

• moves types from the sequencer crate
• Moved SeqTypes to the types crate.
• implements all the hotshot traits for the SeqTypes
• moves all the helper functions, types and some traits needed for hotshot trait implementations or types
• creates a versioned Header type in v0/header.rs and the serialization/deserialization

things to review:

• v0/header.rs
• Serialization and test_versioned_header_serialization() in v0/impls/header.rs
• check that all types and implementations are organized clearly within their respective modules.