rootfs: pseudo-implementation of rootfs unpacking
rootfs: pseudo-implementation of rootfs unpacking

This defines a new package for assembling rootfs with layers and
snapshots. We define some base types and operations that represent the
boiled down process of the unpacking a rootfs.

This code is untested but provides a base to work towards a full
implementation of rootfs unpacking.

Signed-off-by: Stephen J Day <63efd86fa22882ae89b6027b22a36e4db1f4214d@docker.com>

miaoyq/containerd,chanezon/containerd,estesp/containerd,rajasec/containerd,yuexiao-wang/containerd,crosbymichael/containerd,estesp/containerd,miaoyq/containerd,thaJeztah/containerd,Wolphin-project/containerd,mikebrow/containerd,hqhq/containerd,thaJeztah/containerd,dmcgowan/containerd,dmcgowan/containerd,containerd/containerd,crosbymichael/containerd,lichao-mobanche/containerd,kunalkushwaha/containerd,docker/containerd,mikebrow/containerd,vdemeester/containerd,AkihiroSuda/containerd,kunalkushwaha/containerd,vdemeester/containerd,containerd/containerd,xiekeyang/containerd