build unixfsv1 DAG from ipldstore data

[d70-t]

The data structures generated by ipldstore could be translated into unixfsv1 structures sharing the same leaves (i.e. by using the --raw-leaves feature).

The unixfsv1 tree could be built on a machine without touching or downloading any chunk-data, so requires much less memory. But producing the unixfsv1 tree would make it possible to access data gererated using this method via gateways as well as via ipfs fuse-mounts.