Nix Integration

[mara-schulke]

This branch reassembles the changes done by @j-baker on buffrs 0.7.1 for nix support but used the latest buffrs version as base branch.

Introduces

• buffrs::cache – a filesystem cache for packages downloaded during the installation (takes precedence over network requests)
• The BUFFRS_CACHE environment variable to set the cache directory
• buffrs lock print-files – prints the files that buffrs requires to be downloaded as JSON to stdout
• buffrs publish --version – the version option can be used to override the manifests version while publishing
• buffrs package --version – the version option can be used to override the manifests version while packaging
• A nix implementation that populates the buffrs cache

Removes

• buffrs generate – remove the code generation command
• buffrs::build! – remove the build macro from buffrs due to compatibility issues
• buffrs::generate – remove the code generation implementation

Updates

• Documentation on the above

---

The contents of this PR are based entirely of #172