pgsql: Move nbtree preprocessing into new .c file.

Move nbtree preprocessing into new .c file.

Quite a bit of code within nbtutils.c is only called during nbtree
preprocessing. Move that code into a new .c file, nbtpreprocesskeys.c.
Also reorder some of the functions within the new file for clarity.

This commit has no functional impact. It is strictly mechanical.

Author: Peter Geoghegan <pg(at)bowt(dot)ie>
Suggested-by: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Discussion: https://postgr.es/m/CAH2-WznwNn1BDOpWxHBUK1f3Rdw8pO9UCenWXnvT=n9GO8GnLA@mail.gmail.com
Discussion: https://postgr.es/m/86930045-5df5-494a-b4f1-815bc3fbcce0%40iki.fi

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/597b1ffbf12352a3863a894f16741864aaf2242f

Modified Files
--------------
src/backend/access/nbtree/Makefile | 1 +
src/backend/access/nbtree/meson.build | 1 +
src/backend/access/nbtree/nbtpreprocesskeys.c | 1864 +++++++++++++++++++
src/backend/access/nbtree/nbtutils.c | 2374 +++----------------------
src/include/access/nbtree.h | 11 +-
5 files changed, 2135 insertions(+), 2116 deletions(-)