Make protected::traits public so that I can make types using Protected<>

I cannot use Protected memory in any of my types because the `traits` module is private, so I could not actually write something like:

```rs
struct ProtectedMemory {
  my_struct: Protected<HeapBytes, ReadOnly, Locked>
}
```

src/protected.rs

@@ -101,7 +101,8 @@ mod int {
  }
 }
 
-mod traits {
+#[doc(hidden)] // Edit this PR to remove doc(hidden) or add a doc comment.
+pub mod traits {
  pub trait ProtectMode {}
  pub struct ReadOnly {}
  pub struct ReadWrite {}