Rollup merge of #112628 - gootorov:box_alloc_partialeq, r=joshtriplett

Allow comparing `Box`es with different allocators

Currently, comparing `Box`es over different allocators is not allowed:
```Rust
error[E0308]: mismatched types
  --> library/alloc/tests/boxed.rs:22:20
   |
22 |     assert_eq!(b1, b2);
   |                    ^^ expected `Box<{integer}, ConstAllocator>`, found `Box<{integer}, AnotherAllocator>`
   |
   = note: expected struct `Box<{integer}, ConstAllocator>`
              found struct `Box<{integer}, AnotherAllocator>`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `alloc` (test "collectionstests") due to previous error
```
This PR lifts this limitation

Author: matthiaskrgr