Motivation

This puts MetaOCaml on more equal footing with the normal OCaml
packages, which have a few passthru's and expect to have more 'meta'
information available (such as 'platforms').

With these changes, you can use the ber_metaocaml package along with
ocaml-ng.mkOcamlPackages in order to create a full package set with
Native MetaOCaml support, though this currently isn't implemented (you
have to do this yourself).

There are also other light cleanups, for example this also removes the
old MIPS support and restricts the platforms to x86 Linux/Darwin, for
now. Other platforms can be added on a case-by-case basis.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.