implement lazy fetch support

[tych0]

It would be nice to have lazy fetch. I think this would happen in a couple of steps:

• abstract reader::fuse::Filesystem to take something like umoci's CAS (although maybe with a clearer name :D)
• implement a lazy-fetching CAS which knows about both an on disk location like oci::Image does now, but also a repo to go fetch blobs from if it doesn't have them

[ariel-miculas]

It's unclear how we can implement this in the kernel module or if it's useful just as a feature for the fuse version.