reimplement unpacking ourselves

[cyphar]

The current upstream unpacking utilities have some flaws:

• They are completely separate to my CAS implementation, making using them quite ugly.
• They don't expose a "single layer" unpacking method, meaning that we can't implement raw-unpack (umoci: implement raw-unpack #23) and we can't really test it properly.
• They have bugs (like unpack: doesn't preserve mtime #1) and also don't support things like "user:group"-style specification of users, which is a problem.
• Features like *pack: implement rootless unpacking/repacking #26 are unlikely to be nicely solved upstream.

All of this can be solved if we just implement it ourselves.

[cyphar]

This is being worked on in https://github.com/cyphar/umoci/tree/layer-unpacking.

[cyphar]

Done in 7654ebc.