add Task.completed/1

[superhawk610]

This PR implements the new Task function Task.completed/1 as proposed in the elixir-lang group.

This function models a task that immediately completes with a known result. As documented in the @doc block, it's a useful convenience when working with groups of asynchronous tasks where some inputs may have a result that can be determined without actually running the task. Here's a quick example:

def process(data) do
  tasks =
    for entry <- data do
      if invalid_input?(entry) do
        Task.completed({:error, :invalid_input})
      else
        Task.async(fn -> further_process(entry) end)
      end
    end

  Task.await_many(tasks)
end

Here are a couple links to prior art in .NET:

https://docs.microsoft.com/en-us/dotnet/api/system.threading.tasks.task.completedtask?view=net-5.0

https://docs.microsoft.com/en-us/dotnet/api/system.threading.tasks.task.fromresult?view=net-5.0

Credit for the idea goes to @lukebakken.

[lukebakken]

Great!

[josevalim]

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