Replace sha256 with hash in documentation and all-packages.nix

[nbraud]

Description of changes

• Add a new sha256-to-SRI.py maintainer script, which is both useable as a library, or as an executable which attempts to replace sha256 attributes with hash ones.

• Replace deprecated uses of sha256 with hash in the documentation: both nixpkgs', NixOS', and comments under lib/.

  Remaining occurences of sha256 under doc/ are in the context of functions which do not yet accept hash parameters:

  • BEAM, Dart, and Coq language frameworks;
  • Docker image builder.

• Replace uses of sha256 with hash in all-packages.nix

Things done

• Ran mypy on the new maintainer script
• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Fits CONTRIBUTING.md.