utf8.c: Speed up \X processing of Korean

utf8.c: Speed up \X processing of Korean

\X matches according to a complicated pattern that is hard-coded in
regexec.c.  Part of that pattern involves checking if a code point is a
component of a Hangul Syllable or not.  For Korean code points, this
involves checking against multiple tables.  It turns out that two of
those tables are arranged so that the checks for them can be done via an
arithmetic expression; Unicode publishes algorithms for determining
various characteristics based on their very structured ordering.

This patch converts the routines that check these two tables to instead
use the arithmetic expression.