Use platform independent type for TupleTableSlot->tts_off.

 Use platform independent type for TupleTableSlot->tts_off.
 
 Previously tts_off was, for unknown reasons, of type long. For one
 that's unnecessary as tuples are restricted in length, for another
 long would be a bad choice of type even if that weren't the case, as
 it's not reliably wider than an int. Also HeapTupleHeader->t_len is a
 uint32.
 
 This is split off from a larger patch implementing JITed tuple
 deforming. Seems like an independent improvement, as tiny as it is.
 
 Author: Andres Freund