Fix proof visibility

[vivianjeng]

Description

• Proof elements cannot be used by other crates

  field `message` of struct `SemaphoreProof` is private

  It cannot even check if the message, scope,... match the user's claim

• Also add serde for SemaphoreProof and add tests for it

Related Issue(s)

Other information

Checklist

• I have read and understand the contributor guidelines and code of conduct.
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have run yarn style without getting any errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Important

We do not accept minor grammatical fixes (e.g., correcting typos, rewording sentences) unless they significantly improve clarity in technical documentation. These contributions, while appreciated, are not a priority for merging. If there is a grammatical error feel free to message the team.

[vplasencia]

Thank you very much @vivianjeng!