trie: one node per rune, children in rune order, and the last path kept

The trie walked a key byte by byte, so a Hangul syllable cost three
nodes and every lookup first spelled its key back into UTF-8.  It walks
runes now: a third of the nodes for Korean data, and no buffer in the
lookup.  Children are kept in rune order, so both the search and the
insert stop early, and a trie remembers the path of the last key put,
which a file sorted by key — hanja.dict is one — walks straight down.

dict.c

@@ -24,7 +24,7 @@ words(Tnode *nd, Str *out, int n, int max)
 	return n;
 }
 
-/* The entries at and below a node, in the dictionary's order. */
+/* The entries at and below a node, the shortest keys first. */
 static int
 below(Trie *t, int ni, Str *out, int n, int max)
 {