Cert::insert_packets is slow

See this issue: sequoia-octopus-librnp#25 (closed)

The problem is that the current implementation of Cert::insert_packets is O(n*m) where n is the number of signatures in the certificate and m is the number of signatures in the packets to insert.

This change makes it O(n log n): https://gitlab.com/sequoia-pgp/sequoia/-/commits/neal/smarter-cert-insert-packets .

@teythoon: This needs review since it introduces a new public API.