Use ATEND macros to add fields to abstract types.

This means we can add new properties to abstract types without
breaking old code. We can also make simple abstract types without
needing to add many NULL fields to the type.

src/core/io.c

@@ -47,13 +47,7 @@ JanetAbstractType cfun_io_filetype = {
     cfun_io_gc,
     NULL,
     io_file_get,
-    NULL,
-    NULL,
-    NULL,
-    NULL,
-    NULL,
-    NULL,
-    NULL
+    JANET_ATEND_GET
 };
 
 /* Check arguments to fopen */