Migrate IOFile implementation to io_file.cpp

Makes the implementation of `IOFile` private, allowing inclusions and
defines such as `#define fseeko` and `#include <io.h>` to not poison
client-code or the global namespace.

CMakeLists.txt

@@ -67,7 +67,7 @@ else()
     message(FATAL_ERROR "Currently unsupported CPU architecture")
 endif()
 
-set(SOURCE_FILES src/main.cpp src/emulator.cpp src/core/CPU/cpu_dynarmic.cpp src/core/CPU/dynarmic_cycles.cpp
+set(SOURCE_FILES src/main.cpp src/emulator.cpp src/io_file.cpp src/core/CPU/cpu_dynarmic.cpp src/core/CPU/dynarmic_cycles.cpp
                  src/core/memory.cpp
 )
 set(KERNEL_SOURCE_FILES src/core/kernel/kernel.cpp src/core/kernel/resource_limits.cpp