Investigate `tarfs` support (as part of image support)

This came up during work on https://github.com/hermit-os/uhyve/issues/1015.

This would be a way to reduce possible future code duplication / duplicated work between `loader` and `uhyve`.
The main alternative to this is:
* `uhyve` implements "virtual file descriptors" to refer to read-only segments / blobs / slices of (decompressed tar) files. (this is now done, but mostly because it was intended to get done even independently of this)
* `loader` implements a way to parse tar files and pass the file tree via FDT (device tree) to the kernel, the kernel would then handle this as a read-only file system. (this idea was ultimately rejected in favor of this)

cc @n0toose