Unionvisor systemd NixOS module and `Bundle` system refactor (closes #21) (#27)

- Refactors the `Bundle` (formerly: `Bindir`). system. Bundles are now
fully immutable such that they can live in the `/nix/store`, and the
`bins/` directory no longer contains a `current` symlink.
- The `current` symlink is now instead handled by `Symlinker`, which
asserts validity of binaries in a bundle, and then creates a `current`
link in the `--root`, fully decoupled from the bundle itself.
- Bundles now contian a `meta.json`, which describes their structure. 
- We've decided to fully remove the `revert` system
- Since data in `config/` is also mutated by `uniond`, we now backup the
entire `home/` dir.
- Updates naming to avoid confusion between the _unionvisor root_ and
the _uniond home_. The former is now called `root` everywhere. (Prev. it
was called `home` in some cases)
- Various small fixes (typos such as "monniker")
- Adds `cargo-nextest` to the devShell
- Bumps workspace rust channel to `nightly-2023-05-16`
- Fixes `clippy::pedantic` warnings for the entire crate

I'm planning on exposing more configuration options to the
`nixosModule`, but this PR is already quite big so this will be done in
a future PR.