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.s

@@ -0,0 +1,32 @@
+.include "io.inc65"
+
+.importzp sp, sreg
+
+.export _mapper_enable
+.export _mapper_read, _mapper_write
+
+.autoimport on
+
+.code
+
+
+; void mapper_enable(uint8_t en)
+_mapper_enable:
+	sta MM_CTRL
+	rts
+
+_mapper_read:
+	phx
+	tax
+	lda MM_DATA,x
+	ldx #$00
+	rts
+
+_mapper_write:
+	phx
+	tax
+	jsr popa
+	sta MM_DATA,x
+	plx
+	rts
+