Implement builtins.zipAttrsWith

[Anton-Latukha]

A port of lib function added in Nix 2.6. Nixpkgs lib/attrsets.nix mentions:

{
  /* Merge sets of attributes and use the function f to merge attributes
     values.

     Example:
       zipAttrsWithNames ["a"] (name: vs: vs) [{a = "x";} {a = "y"; b = "z";}]
       => { a = ["x" "y"]; }
  */
  zipAttrsWithNames = names: f: sets:
    listToAttrs (map (name: {
      inherit name;
      value = f name (catAttrs name sets);
    }) names);

  /* Implementation note: Common names appear multiple times in the list of
     names, hopefully this does not affect the system because the maximal
     laziness avoid computing twice the same expression and listToAttrs does
     not care about duplicated attribute names.

     Example:
       zipAttrsWith (name: values: values) [{a = "x";} {a = "y"; b = "z";}]
       => { a = ["x" "y"]; b = ["z"] }
  */
  zipAttrsWith =
    builtins.zipAttrsWith or (f: sets: zipAttrsWithNames (concatMap attrNames sets) f sets);
}