Update preprocess: now returns generator objects

bench/text.py

@@ -0,0 +1,10 @@
+from timeit import timeit
+
+if __name__ == '__main__':
+
+    res = timeit(
+        setup="from hexlib.text import preprocess",
+        stmt='text = "x A b c d e f g h"\ncleaned = preprocess(\n    text,\n    lowercase=True,\n    trigrams={\n        ("a", "b", "c"),\n        ("e", "f", "g"),\n    }\n)'
+    )
+
+    print(res)