ASL `done` event

[trentgill]

returns the done user-definable event which was removed with ASL2.

this allows the scripter to trigger some event when their ASL construct has completed by redefining the output[n].done function. this is particularly useful now that ASL runs asynchronously from the Lua environment.

output[1].done = function()
  print 'hey the ASL is done!'
end

output[1].slew = 1
output[1].volts = 2

-- <wait 1 second>
--> "hey the ASL is done!"

///

previously there was also a state variable output[n].running which would return a boolean stating whether ASL was active on a channel. i'm unconvinced this is really useful, and also very easy to manage directly by using the above done event.

///

reference note: this PR also removes the deprecated go_toward and LL_toward which are no longer used since ASL2. additionally it optimizes stack usage inside of Casl, by switching a recursive call to a while loop.

[tehn]

added to reference docs

[trentgill]

just found a potential bug here.

early return at line 346 in Casl.c needs to trigger the event too. find a way to unify the control flow.

[trentgill]

solved the above issue using a GOTO :see_no_evil: