Ignore dead_code warnings for tuple structs

```
warning: field `0` is never read
   --> crossbeam-epoch/src/collector.rs:290:21
    |
290 |         struct Elem(i32);
    |                ---- ^^^
    |                |
    |                field in this struct
    |
    = note: `#[warn(dead_code)]` on by default
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
    |
290 |         struct Elem(());
    |                     ~~

warning: field `0` is never read
   --> crossbeam-epoch/src/collector.rs:354:21
    |
354 |         struct Elem(i32);
    |                ---- ^^^
    |                |
    |                field in this struct
    |
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
    |
354 |         struct Elem(());
    |                     ~~

warning: field `0` is never read
   --> crossbeam-epoch/src/collector.rs:431:21
    |
431 |         struct Elem(i32);
    |                ---- ^^^
    |                |
    |                field in this struct
    |
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
    |
431 |         struct Elem(());
    |                     ~~

warning: field `0` is never read
 --> crossbeam-utils/tests/atomic_cell.rs:9:22
  |
9 |     struct UsizeWrap(usize);
  |            --------- ^^^^^
  |            |
  |            field in this struct
  |
  = note: `#[warn(dead_code)]` on by default
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
  |
9 |     struct UsizeWrap(());
  |                      ~~

warning: field `0` is never read
  --> crossbeam-utils/tests/atomic_cell.rs:10:19
   |
10 |     struct U8Wrap(bool);
   |            ------ ^^^^
   |            |
   |            field in this struct
   |
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
   |
10 |     struct U8Wrap(());
   |                   ~~

warning: field `0` is never read
  --> crossbeam-utils/tests/atomic_cell.rs:11:20
   |
11 |     struct I16Wrap(i16);
   |            ------- ^^^
   |            |
   |            field in this struct
   |
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
   |
11 |     struct I16Wrap(());
   |                    ~~

warning: field `0` is never read
  --> crossbeam-utils/tests/atomic_cell.rs:13:22
   |
13 |     struct U64Align8(u64);
   |            --------- ^^^
   |            |
   |            field in this struct
   |
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
   |
13 |     struct U64Align8(());
   |                      ~~

warning: field `0` is never read
 --> crossbeam-channel/benchmarks/message.rs:6:27
  |
6 | pub(crate) struct Message(pub(crate) [usize; LEN]);
  |                   ------- ^^^^^^^^^^^^^^^^^^^^^^^
  |                   |
  |                   field in this struct
  |
  = note: `Message` has a derived impl for the trait `Clone`, but this is intentionally ignored during dead code analysis
  = note: `#[warn(dead_code)]` on by default
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
  |
6 | pub(crate) struct Message(());
  |                           ~~
```

crossbeam-channel/benchmarks/message.rs

@@ -3,7 +3,7 @@ use std::fmt;
 const LEN: usize = 1;
 
 #[derive(Clone, Copy)]
-pub struct Message(pub [usize; LEN]);
+pub struct Message(#[allow(dead_code)] [usize; LEN]);
 
 impl fmt::Debug for Message {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {

crossbeam-epoch/src/collector.rs

@@ -286,7 +286,7 @@ mod tests {
         const COUNT: usize = 100_000;
         static DROPS: AtomicUsize = AtomicUsize::new(0);
 
-        struct Elem(i32);
+        struct Elem(#[allow(dead_code)] i32);
 
         impl Drop for Elem {
             fn drop(&mut self) {
@@ -350,7 +350,7 @@ mod tests {
         const COUNT: usize = 700;
         static DROPS: AtomicUsize = AtomicUsize::new(0);
 
-        struct Elem(i32);
+        struct Elem(#[allow(dead_code)] i32);
 
         impl Drop for Elem {
             fn drop(&mut self) {
@@ -427,7 +427,7 @@ mod tests {
         const COUNT: usize = 100_000;
         static DROPS: AtomicUsize = AtomicUsize::new(0);
 
-        struct Elem(i32);
+        struct Elem(#[allow(dead_code)] i32);
 
         impl Drop for Elem {
             fn drop(&mut self) {

crossbeam-utils/tests/atomic_cell.rs

@@ -6,11 +6,11 @@ use crossbeam_utils::atomic::AtomicCell;
 
 #[test]
 fn is_lock_free() {
-    struct UsizeWrap(usize);
-    struct U8Wrap(bool);
-    struct I16Wrap(i16);
+    struct UsizeWrap(#[allow(dead_code)] usize);
+    struct U8Wrap(#[allow(dead_code)] bool);
+    struct I16Wrap(#[allow(dead_code)] i16);
     #[repr(align(8))]
-    struct U64Align8(u64);
+    struct U64Align8(#[allow(dead_code)] u64);
 
     assert!(AtomicCell::<usize>::is_lock_free());
     assert!(AtomicCell::<isize>::is_lock_free());