Consider adding sugar for the elect() & return single_result idiom

[jaredhoberock]

The idea would be to provide a shorthand for this idiom:

auto lambda = [](auto& self) -> single_result<int>
{
  // each agent computes some result
  int my_result = compute_my_result(self);

  // only one agent needs to return a result
  if(self.elect())
  {
    return my_result;
  }

  return ignore;
}

With some sugar we could have something like:

auto lambda = [](auto& self)
{
  // only one agent needs to return a result
  return elect_value(self, compute_my_result(self));
}

What's nice about the sugar in this example is that it eliminates the requirement to annotate the return type of the lambda with single_result<int>.

[jaredhoberock]

If we do this, we might consider making the current .elect() member function a free function.