More check on nesting AttributeSavingMixin

[toslunar]

This PR improves #134 so that save and load detect infinite loops of length >= 2.

[muupan]

Thank you and sorry for late response. Can you add a test case to check it can detect an infinite loop?

[toslunar]

Added tests to detect loops.

I confirmed the current master fails the new test.

.E..
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ERROR: test_loop2 (tests.test_agent.TestAttributeSavingMixin)
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Traceback (most recent call last):
  File "/Users/tos/Documents/chainerrl/tests/test_agent.py", line 99, in test_loop2
    parent1.save(dirname)
  File "/Users/tos/Documents/chainerrl/chainerrl/agent.py", line 119, in save
    attr_value.save(os.path.join(dirname, attr))
  File "/Users/tos/Documents/chainerrl/chainerrl/agent.py", line 119, in save
    attr_value.save(os.path.join(dirname, attr))
  File "/Users/tos/Documents/chainerrl/chainerrl/agent.py", line 119, in save
    attr_value.save(os.path.join(dirname, attr))
  [Previous line repeated 154 more times]
  File "/Users/tos/Documents/chainerrl/chainerrl/agent.py", line 123, in save
    getattr(self, attr))
  File "/Users/tos/Documents/chainer/chainer/serializers/npz.py", line 73, in save_npz
    with open(file, 'wb') as f:
OSError: [Errno 63] File name too long: '/var/folders/zk/2tl6l77170d2b51fczxysjsw0000gn/T/tmpsawjvmga/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/child/link.npz'

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Ran 4 tests in 0.530s

FAILED (errors=1)

[muupan]

LGTM