Rust variance

[marioortizmanero]

TODO list:

• [ ] Create an issue for the comments
• [ ] Add at least one image for previews when sharing
• [ ] Make sure it doesn't start with section such as "Introduction" (it'll mess up the post preview)
• Add the following metadata fields:
  • [ ] title
  • [ ] description
  • [ ] author
  • [ ] images
  • [ ] tags
  • [ ] series (optionally)
  • [ ] date
  • [ ] GHissueID
• [ ] Clean up bibliography
• [ ] Share on other platforms (lobste.rs, reddit, hackernews, twitter?)