Add mapper 594 with expansion sound.

src/boards/fifo.h

@@ -0,0 +1,19 @@
+#ifndef _FIFO_H
+#define _FIFO_H
+
+typedef struct {
+	size_t capacity;
+	uint8 *data;
+	int16 front;
+	int16 back;
+} FIFO;
+
+size_t FIFO_size (FIFO *);
+uint8 FIFO_halfFull (FIFO *);
+int FIFO_retrieve (FIFO *);
+void FIFO_add (FIFO *, uint8);
+void FIFO_reset (FIFO *);
+void FIFO_init (FIFO *, size_t);
+void FIFO_close (FIFO *);
+
+#endif