lib: refactor lib.attrsets.{mapAttrs', mapAttrsToList} and mapAttrs'' in the strongswan-swanctl module

The new implementation of `mapAttrsToList` is simpler than the previous one, avoiding an extra string conversion. Benchmarking shows a slight performance improvement. See the discussion here: https://discourse.nixos.org/t/another-implementation-of-mapattrstolist

Additionally, I searched nixpkgs for expressions equivalent to the old `mapAttrsToList` and replaced them with direct calls to the new implementation.

lib/attrsets.nix

@@ -1042,7 +1042,7 @@ rec {
 
     :::
   */
-  mapAttrs' = f: set: listToAttrs (map (attr: f attr set.${attr}) (attrNames set));
+  mapAttrs' = f: set: listToAttrs (mapAttrsToList f set);
 
   /**
     Call a function for each attribute in the given set and return
@@ -1076,7 +1076,7 @@ rec {
 
     :::
   */
-  mapAttrsToList = f: attrs: map (name: f name attrs.${name}) (attrNames attrs);
+  mapAttrsToList = f: attrs: attrValues (mapAttrs f attrs);
 
   /**
     Deconstruct an attrset to a list of name-value pairs as expected by [`builtins.listToAttrs`](https://nixos.org/manual/nix/stable/language/builtins.html#builtins-listToAttrs).

nixos/modules/services/networking/strongswan-swanctl/param-lib.nix

@@ -89,7 +89,7 @@ rec {
     in
     recurse [ ] set;
 
-  mapAttrs'' = f: set: foldl' (a: b: a // b) { } (map (attr: f attr set.${attr}) (attrNames set));
+  mapAttrs'' = f: set: foldl' (a: b: a // b) { } (mapAttrsToList f set);
 
   # Extract the options from the given set of parameters.
   paramsToOptions = ps: mapParamsRecursive (_path: name: param: { ${name} = param.option; }) ps;