Remove #[transparent] on RestoreState.

For some reason 1.54 doesn't like it.

```
error[E0690]: transparent struct needs exactly one non-zero-sized field, but has 0
   --> src/lib.rs:123:1
    |
123 | pub struct RestoreState(RawRestoreState);
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ needs exactly one non-zero-sized field, but has 0
```

src/lib.rs

@@ -118,7 +118,6 @@ pub type RawRestoreState = RawRestoreStateInner;
 ///
 /// User code uses [`RestoreState`] opaquely, critical section implementations
 /// use [`RawRestoreState`] so that they can use the inner value.
-#[repr(transparent)]
 #[derive(Clone, Copy, Debug)]
 pub struct RestoreState(RawRestoreState);