Extract tempfile module from fileio

Though this module is relatively small it has very clear boundaries.
The last argument for extracting `tempfile` was the errors which I got
when I was writing unittests for it: `cimport './src/nvim/fileio.h'`
does not work for some reason.

src/nvim/diff.c

@@ -30,6 +30,7 @@
 #include "nvim/path.h"
 #include "nvim/screen.h"
 #include "nvim/strings.h"
+#include "nvim/tempfile.h"
 #include "nvim/undo.h"
 #include "nvim/window.h"
 #include "nvim/os/os.h"