Support tuple data communication

[levelfour]

The current version of chainermn.functions.{send,recv} can communicate with only one array.
This hinders the flexibility of MultiNodeChainList since we can't send multiple arrays at the same time:

class NN0(chainer.Chain):
    def __call__(self, x):
        y0 = some_calculation_nn0_0(x)
        y1 = some_calculation_nn1_1(x)
        return y0, y1

class NN1(chainer.Chain):
    def __call__(self, y):
        y0, y1 = y
        return some_calculation_nn1(y0, y1)

class Model_on_Process_0(chainermn.MultiNodeChainList):
    def __init__(self, comm):
        super(Model_on_Process_0, self).__init__(comm=comm)
        self.add_link(NN0(), rank_in=None, rank_out=1)

class Model_on_Process_1(chainermn.MultiNodeChainList):
    def __init__(self, comm):
        super(Model_on_Process_1, self).__init__(comm=comm)
        self.add_link(NN1(), rank_in=0, rank_out=None)