Want D script to format slog output

[bnaecker]

We use slod-dtrace to fire DTrace USDT probes at every log message, even those which are compiled into the binary but filtered by the level-filter. This is really useful, but the formatting of these messages is a bit funky. To send them to DTrace, we JSON-serialize them, which requires handling the possibility that that can fail. So you get messages in DTrace that are like {"ok": {<actual JSON>} or {"err": "some error message"}.

We should write a D script that will handle these serialization details for us, and print out the actual messages. Ideally, that would go into the Bunyan format that we use everywhere else, so we can pipe the output to tools like looker. We should also be sure to ship these in the software images we build. These probes have very rarely changed, so putting the script in the GZ seems fine for now.