Split tuplesortvariants.c from tuplesort.c

Split tuplesortvariants.c from tuplesort.c

This commit puts the implementation of Tuple sort variants into the separate
file tuplesortvariants.c.  That gives better separation of the code and
serves well as the demonstration that Tuple sort variant can be defined outside
of tuplesort.c.

Discussion: http://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