Adding an interface to manually control the buffering for generated
parsers.

This consists of two parts:

    1. The generated Flow classes expose their flow buffers via a new
       method flow_buffer().

    2. Flow buffers get two new methods:

        // Interface for delayed parsing. Sometimes BinPAC doesn't get the
        // buffering right and then one can use these to feed parts
        // individually and assemble them internally. After calling
        // FinishBuffer(), one can send the uppper-layer flow an FlowEOF()
        // to trigger parsing.
        void BufferData(const_byteptr data, const_byteptr end);
        void FinishBuffer();