Introduce mina_block package instead of mina_transition

[georgeee]

This PR rearranges module structure and finalizes block header/body split work.

Explain how you tested your changes:

• Existing suite is used to prevent regressions
• No change to functionality is made

Checklist:

• Modified the current draft of release notes with details on what is completed or incomplete within this project
• Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• All tests pass (CI will check this if you didn't)
• Serialized types are in stable-versioned modules
• Does this close issues? Separate block headers from block bodies in OCaml #9455