Better human readable representation for PickableSequentialChain

[yuyu2172]

Since chainer/chainer#4853, print message of links became much more readable.

For chains, the order of child links is arbitrary when printed.
PickableSequentialChain has innate sequential nature, which should be reflected on the printed message.
This PR fixes this problem.

Before this PR

from chainercv.links import ResNet50
l = ResNet50()
print(l)

# displayed text
ResNet50(
  (conv1): Conv2DBNActiv(
    (bn): BatchNormalization(size=64, decay=0.9, eps=2e-05, dtype=float32, use_gamma=True, use_beta=True),
    (conv): Convolution2D(in_channels=None, out_channels=64, ksize=7, stride=(2, 2), pad=(3, 3), nobias=True, dilate=(1, 1), groups=1),
  ),
  (fc6): Linear(in_size=None, out_size=1000, nobias=False),
  (res2): ResBlock(

...

After this PR

# displayed text
ResNet50(
  (conv1): Conv2DBNActiv(
    (bn): BatchNormalization(size=64, decay=0.9, eps=2e-05, dtype=float32, use_gamma=True, use_beta=True),
    (conv): Convolution2D(in_channels=None, out_channels=64, ksize=7, stride=(2, 2), pad=(3, 3), nobias=True, dilate=(1, 1), groups=1),
  ),
  (pool1): self.pool1 = lambda x: F.max_pooling_2d(x, ksize=3, stride=2),
  (res2): ResBlock(