Move heap-specific detoasting logic into a separate function.

The new function, heap_fetch_toast_slice, is shared between
toast_fetch_datum_slice and toast_fetch_datum, and does all the
work of scanning the TOAST table, fetching chunks, and storing
them into the space allocated for the result varlena.

As an incidental side effect, this allows toast_fetch_datum_slice
to perform the scan with only a single scankey if all chunks are
being fetched, which might have some tiny performance benefit.

Discussion: http://postgr.es/m/CA+TgmobBzxwFojJ0zV0Own3dr09y43hp+OzU2VW+nos4PMXWEg@mail.gmail.com