Re-enable recursive remote functions in a limited form.

[robertnishihara]

This should partially address #398. This worked before but was broken because it wasn't being tested in the CI.

However, it's unclear if we should support this because we currently cannot fully support it.

For example, the following works.

@ray.remote
def f():
  return [f.remote()]

f.remote()

However, the following does not work.

def create_f():
  @ray.remote
  def f():
    return [f.remote()]
  return f

create_f().remote()

The following also does not work.

def create_and_call_f():
  @ray.remote
  def f():
    return [f.remote()]

  f.remote()  

create_and_call_f()

Given that we aren't supporting it fully, do we want to partially support it?

@mehrdadn what do you think about this?

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[mehrdadn]

Given that we aren't supporting it fully, do we want to partially support it?

Definitely yes from a user standpoint. It's not going to make anyone's life easier if you don't support it, and it definitely makes people's life easier if it's partially supported. So I would just warn the user that the functionality is incomplete and may or may not ever work perfectly (almost certainly the latter).