ENH: identityOp as equivalent to std::identity (C++20)

- similar to how noOp was defined, but with perfect forwarding

STYLE: combine Swap into stdFoam

STYLE: capitalize FileOp, NegateOp template parameters

applications/test/syncTools/Test-syncTools.C

@@ -614,7 +614,7 @@ void testEdgeFlip2(const polyMesh& mesh, Random& rndGen)
             edgePointCombineOp(),
             PointPair(point::max, point::max),
             edgePointTransformOp(),
-            noOp()
+            identityOp()
         );
 
         forAll(fld, edgeI)