Add test for mutually recursive remote functions.

[robertnishihara]

We can do this now that we export remote functions lazily.

Now that we export remote functions lazily, we don't seem to need the cloudpickle workaround anymore.

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[mehrdadn]

(#453)

[robertnishihara]

As a note to myself, we may be able to move test_recursion.py inside of a normal pytest module in this PR.

[mehrdadn]

Could you refresh my memory what the issue was? I've tried all sorts of examples and can't find a single one whose behavior changed with the workaround. I've tried older versions of CloudPickle as far as 0.5.3 (two years old).

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[edoakes]

LGTM

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