Support for easy parallel execution using async

[GamationOnGithub]

Discussed in Celestecord - the idea was to have a simple ExecuteParallel method that could be used to run all the methods passed to it in parallel, like so:

yield await ExecuteParallel(
  Player.Walk(HorizontalDirection.Right, 48, MapUnit.Pixels),
  ((Level)Level).Camera.MoveSmooth(180, 180, 3)
);

Just putting it in an issue so it isn't forgotten once the main porting stage is over. Would be helpful for those used to the (fairly ugly) parallel execution in Lua Cutscenes