Add symlink support

• Implement symlinks
  • Initfs only has links with absolute paths.
  • Relative links are assumed to be relative to link location.
  • Absolute paths must point into the archived location. E.g. if we archive /home/foo/data and wish the resultant link to point to /scheme/rand, then the absolute link must be /home/foo/data/scheme/rand.
• Move archive_common to a crate within the workspace -- Rust doesn't seem to accept #[path(..)] attributes with .. segments anymore so tests wouldn't build on my machine.
• Improve the test to assert complete filesystem structure.
• Bump to edition 2021
• cargo fmt & clippy