Function serialize_cert 

pub fn serialize_cert(cert: &ProofCertificate) -> String

Serialize a ProofCertificate to a compact text format.

Format:

OXICERT v1
id:<id>
decl:<decl_name>
type_hash:<hex>
proof_hash:<hex>
verified_at:<timestamp>
steps:<count>
<step_0>
<step_1>
...