ENH: add UPstream::subProcs() static method

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

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

  instead of

      for
      (
          int proci = Pstream::firstSlave();
          proci <= Pstream::lastSlave();
          ++proci
      )
      {
          ...
      }

applications/test/FixedList/Test-FixedList.C

@@ -297,12 +297,7 @@ int main(int argc, char *argv[])
     {
         if (Pstream::master())
         {
-            for
-            (
-                int proci = Pstream::firstSlave();
-                proci <= Pstream::lastSlave();
-                ++proci
-            )
+            for (const int proci : Pstream::subProcs())
             {
                 IPstream fromSlave(Pstream::commsTypes::blocking, proci);
                 FixedList<label, 2> list3(fromSlave);