lib: Add auxiliary functions to deal with flags

lib/flags.nix

@@ -0,0 +1,24 @@
+{ nixpkgs, ... }:
+
+let
+  inherit (builtins) any filter isList isString stringLength;
+  inherit (nixpkgs.lib.strings) addContextFrom;
+
+  split = strOrList:
+    if isList strOrList then strOrList
+    else builtins.split "[[:space:]]+" strOrList
+      |> filter (flag: isString flag && stringLength flag > 0)
+      |> map (flag: addContextFrom strOrList flag);
+
+  remerge = strOrList: list:
+    if isList strOrList then list
+    else toString list;
+in {
+  remove = rem: strOrList: split strOrList
+    |> filter (flag: !any (elem: flag == elem) rem)
+    |> remerge strOrList;
+
+  subst = sub: strOrList: split strOrList
+    |> map (flag: sub.${flag} or flag)
+    |> remerge strOrList;
+}