Use the tracing crate for structured logging

[felinira]

Everyone is using tracing now and we are late to the party.

If anyone wants to implement this let me know. The first step would be to migrate from log to the tracing crate in magic-wormhole and then using tracing_subscriber in magic-wormhole-cli to subscribe to tracing messages and define multiple output formats (human readable and json would probably be a good start).

For backwards compatibility we should enable the log feature in tracing. This should allow us to switch to tracing without breaking semver (at least I think so).

As a last step the tracing events could then be annotated with useful fields.