Refactor online recognizer (#250)

* Refactor online recognizer.

Make it easier to support other streaming models.

Note that it is a breaking change for the Python API.
`sherpa_onnx.OnlineRecognizer()` used before should be
replaced by `sherpa_onnx.OnlineRecognizer.from_transducer()`.

sherpa-onnx/csrc/text-utils.h

@@ -4,6 +4,7 @@
 // Copyright      2023  Xiaomi Corporation
 #ifndef SHERPA_ONNX_CSRC_TEXT_UTILS_H_
 #define SHERPA_ONNX_CSRC_TEXT_UTILS_H_
+#include <errno.h>.
 #include <stdlib.h>
 
 #include <limits>