Add a benchmark for Cert::insert_packets
See this issue: sequoia-octopus-librnp#25 (closed)
Create the following benchmarks:
- Create a Certitifcate with 10k third-party certifications (they can be from the same key, but should be different signatures) and merge the cert with itself.
- The same, but have the new version include 1 certification that is not in the original.
- Have 10k different certifications on each version of the same certificate.