Instead of simply string matching a line, we now traverse the packet as
a tree and match the path based on a depth first search.
While traversing, we support logical OR and current depth * wildcard
processed as a component based query expression.
Callee's are adjusted to specifically select the appropriate Issuer at
the correct depth.
Fixes#185
tests/test_sequoia.py:
Add unit tests for `keyring_split()`, `keyring_merge()`,
`packet_split()`, `packet_join()`, `inspect()`, `packet_dump()`,
`packet_dump_field()`, `packet_signature_creation_time()`, and
`latest_certification()`.
