4-phase Handshake Master (FSM)

Three states: IDLE, REQ_HIGH, REQ_LOW. On i_send, latch i_data and go to REQ_HIGH (o_req='1'). When i_ack='1', go to REQ_LOW (o_req='0'). When i_ack='0', return to IDLE. o_busy='1' when not in IDLE.