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/unit_test.c

@@ -118,7 +118,7 @@ static const struct ct_test tests[] = {
 	{ "engine/full-boundary-passthrough", engine_full_boundary_passthrough },
 	{ "dict/candidates", dictionary_candidates },
 	{ "dict/misses", dictionary_misses },
-	{ "dict/emoji-identity", dictionary_emoji_identity },
+	{ "dict/prefix", dictionary_prefix },
 	{ "ipc/masks-modifiers", ipc_masks_modifiers },
 	{ "ipc/control-caret-frames", ipc_control_and_caret_frames },
 	{ "ipc/runtime-path", ipc_runtime_path },