Consider providing conversions from agent types to indices

[jaredhoberock]

Allowing execution agent types to convert to their indices might simplify very simple loops where the execution category of the agent is irrelevant:

A saxpy loop

bulk_invoke(par(n), [=](parallel_agent& self)
{
  auto i = self.index();
  z[i] = a * x[i] + y[i];
});

would become

bulk_invoke(par(n), [=](size_t idx)
{
  z[i] = a * x[i] + y[i];
});

The size of the agent's group would be lost, as well as functionality such as concurrent_agent::wait().

This also seems like it would be a little redundant with Intel's proposed for_loop control structure.