Structured output proof of concept

[euclidianAce]

This introduces an 'event' api where events can be 'emitted' by
commands, which goes through the following pipeline:

• a set of callbacks are registered with each command (similar to
  script hooks) by name
• when an event is emitted, one of these names must be provided
• the data is fed to the registered callback, which returns a Report
  which describes how to log and serialize the data
• this report is taken by the event api and depending on whether the
  structured output flag is set will either
  • serialize the data to json (with a very basic hand rolled encoder)
    and write it to stdout
  • log it with the given Logger

The basic structure is as such:

command.new {
   name = "my-cool-command",
   description = ":D",
   exec = function(): integer
      event.emit("cool_event", { adjective = "cool" })
      event.emit("cool_event", { adjective = "neat" })
      return 0
   end,
   events = {
      cool_event = function(params: {string:any}): event.Report
         return {
            log_format = "This is from a %(adjective) event happening",
            parameters = { adjective = params.adjective },
         },
      end,
   },
}

Which with unstructured output will produce

$ cyan my-cool-command
     Info This is from a cool event happening
     Info This is from a neat event happening

But with structured output will produce

$ cyan --structured-output my-cool-command
{"event":"cool_event","data":{"adjective":"cool"}}
{"event":"cool_event","data":{"adjective":"neat"}}

This commit adds a little bit of this as a proof of concept to the
current build command, but not for everything that build can log.
But running

$ cyan build --update-all --structured-output --no-script

will produce nicely structured output, and some data massaging with jq
is easy enough. The following will produce an array of files that were
written by build

$ cyan build --update-all --structured-output --no-script | jq -n '[inputs | select(.event == "wrote_file") | .data.file]'

[euclidianAce]

Relevant issue #21