korean: ㄳ from two consonants, 가 from a vowel typed first; index jamo by scanning

libhangul, and so fcitx5-hangul, joins two lone consonants into the
compound jong they make (rt → ㄳ, split again by a vowel: rtk → ㄱ사),
and by default reorders a vowel typed before its consonant (kr → 가);
strans committed the first jamo and started over.  Backspace on a lone
ㄳ leaves ㄱ, as on a syllable.

The three jamo index tables and their -1 macros were the cho, jung, and
jong arrays inverted by hand, bounded by a Jrange the callers had to
respect; a linear scan of the arrays says the same in ten lines.

tests/ko_test.c

@@ -25,6 +25,12 @@ korean_sequences(struct ct *t)
 		{ "rr", "ㄱㄱ" },
 		{ "Rk", "까" },
 		{ "rk1", "가1" },
+		{ "rt", "ㄳ" },
+		{ "rtk", "ㄱ사" },
+		{ "rtt", "ㄳㅅ" },
+		{ "kr", "가" },
+		{ "hkr", "과" },
+		{ "kk", "ㅏㅏ" },
 	};
 	Str out;
 	int i;
@@ -81,6 +87,7 @@ korean_backspace(struct ct *t)
 {
 	static const struct { char *before, *after; } cases[] = {
 		{ "ㄱ", "" },
+		{ "ㄳ", "ㄱ" },
 		{ "가", "ㄱ" },
 		{ "과", "고" }, { "괘", "고" }, { "괴", "고" },
 		{ "궈", "구" }, { "궤", "구" }, { "귀", "구" },