Converts between Prolog representation, Stream, and C representation, CStream, of a stream.
A valid Prolog stream.
SP_stream * pointer.
At least one argument must be ground.
stream_code/2 is used when there are input/output related operations
performed on the same stream in both Prolog code and foreign code.
The CStream value can be used as the stream argument to
any of the SP_* functions taking a stream argument.