Gleam implementation

[Dondorio]

Pull request requirements:

• Commits are well written and well organized.
• Commits for a specific implementation should be prefixed with
  the implementation name.
• Github Actions CI passes all checks (including self-host)

Additional requirements if you are adding a new implementation (see FAQ for details):

• Follow incremental structure (no common eval code)
• Add impls/<IMPL>/Dockerfile
• Add impls/<IMPL>/Makefile
• Update IMPLS.yml
• Update Makefile.impls
• Update README.md