dict: lookups take a trie; a prefix search is its own function; no self-key rule

dictlookup took a Lang for two reasons that were not its business: to
know whether to walk below the key (the emoji dictionary) and to drop a
candidate equal to the key except there.  No dictionary lists its key
among its candidates — the rule was left from a first version that put
the reading first itself — so it goes, and the walk is dictprefix(),
called by the emoji search alone.

tests/test.h

@@ -95,7 +95,7 @@ void	engine_randomized_stress(struct ct*);
 void	engine_full_boundary_passthrough(struct ct*);
 void	dictionary_candidates(struct ct*);
 void	dictionary_misses(struct ct*);
-void	dictionary_emoji_identity(struct ct*);
+void	dictionary_prefix(struct ct*);
 void	ipc_masks_modifiers(struct ct*);
 void	ipc_control_and_caret_frames(struct ct*);
 void	ipc_runtime_path(struct ct*);