Overview over install speed improvements

[hegjon]

Known pull requests of experiments:

• Install from a prebuilt rootfs image instead of pacstrapping 925 packages #108
• Install the target from a btrfs root image instead of pacstrapping it #113
• Install with archinstall-bash: moving towards systemd as the orchestrator #132 (depends on 113)
• Speed up installs with compressed qcow2 restores and parallel finalization #145 (depends on 113)