Improve redoxer filesystem generation

Redoxer is currently broken in CI and on many other distros (including mine).

CI is particularly important to fix, especially for relibc running on Redox. Some possible solutions I have in mind (any of these solutions would be sufficient): This crash was fixed by redoxer!22 (merged), but perf is still suboptimal.

• find and fix the redoxfs EIO bug (FUSE frontend), sort of necessary either way, but hard to know how much time this will take (redoxfs#49)
• rewrite pkgar+pkgutils+installer to support a generic filesystem, might temporarily need more RAM than usual (to unpack to /tmp and indirectly to redoxfs) since tar-rs only supports unpacking to OS directories without reimplementing its wrappers. This will improve the flexibility of the installer as it would no longer require FUSE, and the whole thing would likely be faster. I have already "started" trying to implement this, but probably won't prioritize it. (installer#22)
• implement overlayfs and a readonly driver for either squashfs or tar (the latter already being a possible fs for the installer). Necessary for proper livedisk anyway (#1568)
• implement a 9p driver to avoid the filesystem creation step in the first place, except bootloader+bootstrap+initfs? Maybe the ideal solution for CI?