Structured output proof of concept

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]'

cyan-dev-1.rockspec

@@ -29,6 +29,8 @@ build = {
       ["cyan.commands.run"] = "build/cyan/commands/run.lua",
       ["cyan.commands.warnings"] = "build/cyan/commands/warnings.lua",
       ["cyan.config"] = "build/cyan/config.lua",
+      ["cyan.event"] = "build/cyan/event.lua",
+      ["cyan.eventtypes"] = "build/cyan/eventtypes.lua",
       ["cyan.fs.init"] = "build/cyan/fs/init.lua",
       ["cyan.fs.path"] = "build/cyan/fs/path.lua",
       ["cyan.graph"] = "build/cyan/graph.lua",
@@ -51,6 +53,8 @@ build = {
          ["cyan.commands.run"] = "src/cyan/commands/run.tl",
          ["cyan.commands.warnings"] = "src/cyan/commands/warnings.tl",
          ["cyan.config"] = "src/cyan/config.tl",
+         ["cyan.event"] = "src/cyan/event.tl",
+         ["cyan.eventtypes"] = "src/cyan/eventtypes.tl",
          ["cyan.fs.init"] = "src/cyan/fs/init.tl",
          ["cyan.fs.path"] = "src/cyan/fs/path.tl",
          ["cyan.graph"] = "src/cyan/graph.tl",

src/cyan/cli.tl

@@ -5,6 +5,7 @@
 local argparse <const> = require("argparse")
 local tl <const> = require("tl")
 
+local event <const> = require("cyan.event")
 local command <const> = require("cyan.command")
 local common <const> = require("cyan.tlcommon")
 local config <const> = require("cyan.config")
@@ -19,6 +20,9 @@ local keys <const>, from <const>, sort <const>, ivalues <const>
 local parser <const> = argparse("cyan", "The Teal build system")
 parser:add_help(false)
 
+parser:flag("--structured-output", "Make all command output structured json rather than human readable")
+      :action(function() event.set_structured(true) end)
+
 parser:option("-l --preload", "Execute the equivalent of require('modulename') before processing Teal files.")
       :argname("<modulename>")
       :count("*")

src/cyan/command.tl

@@ -9,6 +9,7 @@ local config <const> = require("cyan.config")
 local fs <const> = require("cyan.fs")
 local log <const> = require("cyan.log")
 local util <const> = require("cyan.util")
+local type eventtypes = require("cyan.eventtypes")
 
 local merge_list <const>, sort <const>, from <const>, keys <const>, contains <const>
    = util.tab.merge_list, util.tab.sort_in_place, util.tab.from, util.tab.keys, util.tab.contains
@@ -21,6 +22,7 @@ local record Args
    gen_compat: tl.CompatMode
    gen_target: tl.TargetMode
    quiet: boolean
+   structured_output: boolean
    global_env_def: string
    verbosity: log.Verbosity
 
@@ -50,6 +52,7 @@ local record Command
    argparse: function(argparse.Command)
    script_hooks: {string}
    exec: CommandFn
+   events: {string:eventtypes.Handler}
 end
 local command <const> = {
    running: Command = nil,