change name of internal function to clarify setitem and getitem
change name of internal function to clarify setitem and getitem

kiyukuta/chainer,jnishi/chainer,okuta/chainer,jnishi/chainer,cupy/cupy,ronekko/chainer,niboshi/chainer,wkentaro/chainer,okuta/chainer,ysekky/chainer,okuta/chainer,keisuke-umezawa/chainer,kashif/chainer,jnishi/chainer,hvy/chainer,keisuke-umezawa/chainer,chainer/chainer,okuta/chainer,keisuke-umezawa/chainer,wkentaro/chainer,hvy/chainer,wkentaro/chainer,hvy/chainer,niboshi/chainer,aonotas/chainer,cupy/cupy,jnishi/chainer,ktnyt/chainer,delta2323/chainer,niboshi/chainer,chainer/chainer,tkerola/chainer,anaruse/chainer,wkentaro/chainer,chainer/chainer,cupy/cupy,chainer/chainer,ktnyt/chainer,cupy/cupy,pfnet/chainer,ktnyt/chainer,keisuke-umezawa/chainer,hvy/chainer,ktnyt/chainer,rezoo/chainer,niboshi/chainer