Implemented new memory mapping to improve performance of self modifying code.

New scheme allows code blocks to cross pages.
Reduced code block size down to 2kB.

src/codegen.c

@@ -2,6 +2,7 @@
 
 #include "ibm.h"
 #include "x86_ops.h"
+#include "mem.h"
 #include "codegen.h"
 
 void (*codegen_timing_start)();