home/packages: add forester

this was much too much work. to summarise:
- ocamlc is weird and does not at all play nice with nix
- nixpkgs.ocamlPackages seems to not have any good way to deal with
  multiple package versions, and forester needs several different
  versions than are in nixpkgs
- forester itself has a flake which one is supposed to use with Nix; it
  uses tweag's opam-nix construct
- which for some bizarre reason cannot be imported the way I import all
  my other flakes, and I do not understand why
- so I gave up for now and import it via flake-compat, unfortunately
  tying me to an old nixpkgs pin (~June), which hopefully does not
  influence too much of the output
- forester's flake's default output is a cross-compiled musl version (???)
- so use the one in legacyPackages instead

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