v1.Layer doesn't abstract away tarball

There's not a way to access files in a `v1.Layer` other than through `Uncompressed` + `tar.NewReader`. Our current abstraction is fine for moving images around, but not for accessing the actual filesystem. If we exposed a way to directly access files, we could be resilient to [changes to the format](https://www.cyphar.com/blog/post/20190121-ociv2-images-i-tar), but we probably don't have enough information to make good decisions for the actual abstraction. Opening this as a nice place to dump ideas.