Introduce notion of data type (dtype similarly to NumPy) into pychan and
teach it to accept for send objects only matching that dtype. Likewise
teach pychan to decode raw bytes received from underlying channel into
Python object correspodningg to pychan dtype. For C dtypes, e.g.
'C.int', 'C.double' etc, contrary to chan of python objects, the
transfer can be done without depending on Python GIL. This way channels
of such C-level dtypes can be used to implement interaction in between
Python and nogil worlds.