This is the smallest useful change to add structured logging. It adds little useful logging, but builds a foundation for adding more useful logging later, and for converging existing log messages to structured one later.

Add some acceptance criteria for structured logging so that we do not accidentally change the log messages we really, really care about.

Add dependencies on the slog and slog-json crates, which exist precisely for structured logging.

Signed-off-by: Lars Wirzenius liw@liw.fi