Ignore dead_code lint on Assert* traits

```
warning: trait `AssertSync` is never used
   --> futures-core/src/task/__internal/atomic_waker.rs:209:15
    |
209 |         trait AssertSync: Sync {}
    |               ^^^^^^^^^^
    |
    = note: `#[warn(dead_code)]` on by default

warning: trait `AssertKinds` is never used
   --> futures-channel/src/mpsc/mod.rs:130:7
    |
130 | trait AssertKinds: Send + Sync + Clone {}
    |       ^^^^^^^^^^^
    |
    = note: `#[warn(dead_code)]` on by default

warning: trait `AssertSendSync` is never used
  --> futures-executor/src/thread_pool.rs:48:7
   |
48 | trait AssertSendSync: Send + Sync {}
   |       ^^^^^^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default

warning: trait `AssertSend` is never used
  --> futures-channel/tests/mpsc.rs:13:7
   |
13 | trait AssertSend: Send {}
   |       ^^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default
```

Author: taiki-e

futures-channel/src/mpsc/mod.rs

@@ -127,6 +127,7 @@ pub struct Sender<T>(Option<BoundedSenderInner<T>>);
 /// This value is created by the [`unbounded`] function.
 pub struct UnboundedSender<T>(Option<UnboundedSenderInner<T>>);
 
+#[allow(dead_code)]
 trait AssertKinds: Send + Sync + Clone {}
 impl AssertKinds for UnboundedSender<u32> {}
 

futures-channel/tests/mpsc.rs

@@ -10,6 +10,7 @@ use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::{Arc, Mutex};
 use std::thread;
 
+#[allow(dead_code)]
 trait AssertSend: Send {}
 impl AssertSend for mpsc::Sender<i32> {}
 impl AssertSend for mpsc::Receiver<i32> {}

futures-core/src/task/__internal/atomic_waker.rs

@@ -206,6 +206,7 @@ impl AtomicWaker {
     /// Create an `AtomicWaker`.
     pub const fn new() -> Self {
         // Make sure that task is Sync
+        #[allow(dead_code)]
         trait AssertSync: Sync {}
         impl AssertSync for Waker {}
 

futures-executor/src/thread_pool.rs

@@ -45,6 +45,7 @@ pub struct ThreadPoolBuilder {
     before_stop: Option<Arc<dyn Fn(usize) + Send + Sync>>,
 }
 
+#[allow(dead_code)]
 trait AssertSendSync: Send + Sync {}
 impl AssertSendSync for ThreadPool {}