engine: one trans contract for every language

Japanese was never dispatched through the Lang table: dotrans and
transstr both special-cased it into transjp, which edited Im in place
and wrote commits through a side argument, while transko popped and
cleared a pre that every caller overwrote anyway, and Vietnamese kept
its key history in dotrans. Emit now carries the new raw state next to
the new pre; transjp, transko and transvi are pure functions of (Im,
key), and dotrans and transstr have a single path. Vietnamese history
bookkeeping lives in vi.c, where the full-history flush no longer
resets the caret as a side effect; istone was toneidx() >= 0.

dat.h

@@ -48,12 +48,18 @@ struct Str
 	int	n;
 };
 
+/*
+ * What one key does to a composition: text to commit, the new pending
+ * text, and the new raw state (Telex key history, or pending Japanese
+ * romaji), given the old ones in Im.
+ */
 typedef struct Emit Emit;
 struct Emit
 {
 	int	eat;
 	Str	s;
 	Str	next;
+	Str	raw;
 };
 
 typedef struct Tnode Tnode;