Get rid of fiber for finalizers.

Instead, finalizers just get access to the foreign object's raw bytes.
This is deliberately limiting, since it discourages the user from
interacting with the VM in the middle of a GC.

src/module/io.c

@@ -144,9 +144,9 @@ void fileAllocate(WrenVM* vm)
   *fd = (int)wrenGetSlotDouble(vm, 1);
 }
 
-void fileFinalize(WrenVM* vm)
+void fileFinalize(void* data)
 {
-  int fd = *(int*)wrenGetSlotForeign(vm, 0);
+  int fd = *(int*)data;
   
   // Already closed.
   if (fd == -1) return;