ENH: add List::resize_unsafe(label)

- changes the addressed list size without affecting list allocation.
  Can be useful for the following type of coding pattern:

  - pre-allocate a List with some max content length
  - populate with some content (likely not the entire pre-allocated size)
  - truncate the list to the length of valid content
  - process the List
  - discard the List

  Since the List is being discarded, using resize_unsafe() instead of
  resize() avoids an additional allocation with the new size and
  copying/moving of the elements.

  This programming pattern can also be used when the List is being
  returned from a subroutine, and carrying about a bit of unused memory
  is less important than avoiding reallocation + copy/move.

  If used improperly, it can obviously result in addressing into
  unmanaged memory regions (ie, 'unsafe').

applications/test/List/Test-List.C

@@ -169,8 +169,17 @@ int main(int argc, char *argv[])
             Info<<" " << *iter;
         }
         Info<< nl;
-    }
 
+        Info<< "data:" << Foam::name(ident.cdata())
+            << " size:" << ident.size() << nl;
+
+
+        Info<< "resize_unsafe(10)" << nl;
+        ident.resize_unsafe(10);
+
+        Info<< "data:" << Foam::name(ident.cdata())
+            << " size:" << ident.size() << nl;
+    }
 
     if (false)
     {