support memoryview to char* conversion
As it is possible to pass a Python bytes instance as the value for char* parameter of a C function, it would be nice if it were possible to pass a memoryview instance as well.
This is useful for implementing the socket-like interface, for example. socket.send(memoryview) works, but in order to implement this using cffi, it's necessary to do an extra conversion to bytes first, eg .send(memoryview.tobytes()). If cffi did this it could probably do it with less copying.