hjem: migrate to new linker

[NotAShelf]

This marks the beginning of the migration to the new linker, @Gerg-L's smfh. We want this migration more importantly for the ability to clean up unmanaged files, but also because it is better to have a home-made solution that we retain some control over. Systemd-tmpfiles has proven ineffective, although they do not need to be phased out completely.

TODO:

1. Actual checks
   • Test basic linking functionality
   • Test manifest-based linking functionality (CC @eclairevoyant)
2. Documentation
   • Update README
   • Module options and usage intructions
3. Replace systemd-tmpfiles implementation
   • We probably don't want to fully drop this one. It's likely a better idea to allow choosing the linker, but defaulting to smfh for this. Relevant: Feature request: Darwin/Standalone support. #4
4. Manifest & friends
   • Implement file manifest from files module

Closes #10