This adds SQL expressions NORMALIZE() and IS NORMALIZED to convert and
check Unicode normal forms, per SQL standard.

To support fast IS NORMALIZED tests, we pull in a new data file
DerivedNormalizationProps.txt from Unicode and build a lookup table
from that, using techniques similar to ones already used for other
Unicode data.  make update-unicode will keep it up to date.  We only
build and use these tables for the NFC and NFKC forms, because they
are too big for NFD and NFKD and the improvement is not significant
enough there.
Reviewed-by: Daniel Verite <daniel@manitou-mail.org>
Reviewed-by: Andreas Karlsson <andreas@proxel.se>
Discussion: https://www.postgresql.org/message-id/flat/c1909f27-c269-2ed9-12f8-3ab72c8caf7a@2ndquadrant.com