[Deliverable] Rework Zerokit Public API

[jm-clius]

Owner: AnonComms Zerokit-RLN

FURPS:

• U1. The Zerokit public API is defined in a specification
• U2. The Zerokit public API provides an easy-to-use interface for developers in Rust and C

Checklist:

• Specs: link to specs and/or API definition
• Code: link to GitHub issues/PRs/Epic
• Dogfood: link to dogfooding session/artefact
• Docs: links to README.md or other docs