Add new Tuplesortstate.removeabbrev function

Add new Tuplesortstate.removeabbrev function

This commit is the preparation to move abbreviation logic into
puttuple_common().  The new removeabbrev function turns datum1 representation
of SortTuple's from the abbreviated key to the first column value.  Therefore,
it encapsulates the differential part of abbreviation handling code in
tuplesort_put*() functions, making these functions similar.

Discussion: https://postgr.es/m/CAPpHfdvjix0Ahx-H3Jp1M2R%2B_74P-zKnGGygx4OWr%3DbUQ8BNdw%40mail.gmail.com
Author: Alexander Korotkov
Reviewed-by: Pavel Borisov, Maxim Orlov, Matthias van de Meent
Reviewed-by: Andres Freund, John Naylor