pgsql: 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: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d0b193c0fad13cf35122b0d3dc805c76e323e8bf

Modified Files
--------------
src/backend/utils/sort/Makefile            |    1 +
src/backend/utils/sort/tuplesort.c         | 1729 +---------------------------
src/backend/utils/sort/tuplesortvariants.c | 1577 +++++++++++++++++++++++++
src/include/utils/tuplesort.h              |  222 +++-
4 files changed, 1784 insertions(+), 1745 deletions(-)