lib.foldl': avoid unnecessary function call

  foldl' =

    op:

    acc:

    list:

    # The builtin `foldl'` is a bit lazier than one might expect.

    # See https://github.com/NixOS/nix/pull/7158.

    # In particular, the initial accumulator value is not forced before the first iteration starts.

    builtins.seq acc

      (builtins.foldl' op acc list);

      (builtins.foldl' op acc);

  /**

    Map with index starting from 0