Fix multiple Default implementation caveats

kernel/aster-nix/src/device/tty/driver.rs

@@ -73,6 +73,12 @@ impl TtyDriver {
     }
 }
 
+impl Default for TtyDriver {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 fn console_input_callback(mut reader: VmReader) {
     let tty_driver = get_tty_driver();
     while reader.remain() > 0 {

kernel/aster-nix/src/events/subject.rs

@@ -85,3 +85,9 @@ impl<E: Events, F: EventsFilter<E>> Subject<E, F> {
         });
     }
 }
+
+impl<E: Events> Default for Subject<E> {
+    fn default() -> Self {
+        Self::new()
+    }
+}

kernel/aster-nix/src/fs/file_table.rs

@@ -198,6 +198,12 @@ impl FileTable {
     }
 }
 
+impl Default for FileTable {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl Clone for FileTable {
     fn clone(&self) -> Self {
         Self {

kernel/aster-nix/src/process/process_vm/heap.rs

@@ -97,3 +97,9 @@ impl Clone for Heap {
         }
     }
 }
+
+impl Default for Heap {
+    fn default() -> Self {
+        Self::new()
+    }
+}