ENH: add UPstream::allProcs() method

- returns a range of `int` values that can be iterated across.
  For example,

      for (const int proci : Pstream::allProcs()) { ... }

  instead of

      for (label proci = 0; proci < Pstream::nProcs(); ++proci) { ... }

applications/utilities/parallelProcessing/redistributePar/redistributePar.C

@@ -185,7 +185,7 @@ void printMeshData(const polyMesh& mesh)
     label totProcPatches = 0;
     label maxProcFaces = 0;
 
-    for (label procI = 0; procI < Pstream::nProcs(); ++procI)
+    for (const int procI : Pstream::allProcs())
     {
         Info<< nl
             << "Processor " << procI << nl