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

src/backend/access/nbtree/Makefile

@@ -17,6 +17,7 @@ OBJS = \
 	nbtdedup.o \
 	nbtinsert.o \
 	nbtpage.o \
+	nbtpreprocesskeys.o \
 	nbtree.o \
 	nbtsearch.o \
 	nbtsort.o \

src/backend/access/nbtree/meson.build

@@ -5,6 +5,7 @@ backend_sources += files(
   'nbtdedup.c',
   'nbtinsert.c',
   'nbtpage.c',
+  'nbtpreprocesskeys.c',
   'nbtree.c',
   'nbtsearch.c',
   'nbtsort.c',