[Deliverable] Rework Zerokit WASM FFI

**Owner**: AnonComms Zerokit-RLN

**FURPS**:

- F4. The Zerokit module supports big-endian operations
- U3. The Zerokit FFI, including WASM targets, supports big-endian arguments

**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