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.

dat.h

@@ -74,7 +74,7 @@ struct Tnode
 	int	vlen;
 	int	child;
 	int	sibling;
-	char	c;
+	Rune	c;
 };
 
 typedef struct Trie Trie;
@@ -83,6 +83,9 @@ struct Trie
 	Tnode	*nodes;
 	int	n;
 	int	cap;
+	/* the last key put and its path, so a sorted file walks it once */
+	Str	last;
+	int	path[Maxrunes];
 };
 
 enum