lib: add xor

This gets clumsily reimplemented in various places, to no useful end.

lib/tests/misc.nix

@@ -102,6 +102,7 @@ let
     types
     updateManyAttrsByPath
     versions
+    xor
     ;
 
   testingThrow = expr: {
@@ -210,6 +211,21 @@ runTests {
     expected = false;
   };
 
+  testXor = {
+    expr = [
+      (xor true false)
+      (xor true true)
+      (xor false false)
+      (xor false true)
+    ];
+    expected = [
+      true
+      false
+      false
+      true
+    ];
+  };
+
   testFix = {
     expr = fix (x: {a = if x ? a then "a" else "b";});
     expected = {a = "a";};