Add memory mapper software interface

Adds functions to read and write mappings, as well as enable and disable
the memory mapper.

This also moves increases the io space by 16 bytes.

sw/mapper.h

@@ -0,0 +1,12 @@
+#ifndef _MAPPER_H
+#define _MAPPER_H
+
+#include <stdint.h>
+
+void mapper_enable(uint8_t en);
+
+uint8_t mapper_read(uint8_t addr);
+void mapper_write(uint8_t data, uint8_t addr);
+
+#endif
+