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