ENH: provide basic hooks for alternative List/Matrix allocators

- include "contiguous.H" in ListPolicy, which makes it available
  whenever UList.H has been included

ENH: return plain List instead of shrinking the DynamicList

applications/utilities/mesh/manipulation/splitMeshRegions/splitMeshRegions.C

@@ -256,7 +256,7 @@ labelList getNonRegionCells(const labelList& cellRegion, const label regionI)
             nonRegionCells.append(celli);
         }
     }
-    return nonRegionCells.shrink();
+    return labelList(std::move(nonRegionCells));
 }