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/tempfile.h

@@ -0,0 +1,8 @@
+#ifndef NVIM_TEMPFILE_H
+#define NVIM_TEMPFILE_H
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "tempfile.h.generated.h"
+#endif
+
+#endif  // NVIM_TEMPFILE_H