This MR will add support for SPG Verein 4.0, fixes #3 (closed)

• A new parser approach that can handle SPG v3 and v4. The parser will be written in Rust because it is more safe than parsing files in PHP. Additionally, it should be more efficient when parsing the larger v4 files.
  • Parsing v3 (Fixes #5 (closed))
  • Parsing v4
  • Use streaming for the parser (maybe async-stream crate)
  • Cross-compiling for Raspbian (see here for reference)
  • Use smallvec instead of heapless
• Sharing SPG files has to work