Ignore invalid_reference_casting lint

I believe this is a false positive. rust-lang/rust#121074

    error: casting references to a bigger memory layout than the backing allocation is undefined behavior, even if the reference is unused
      --> src/arena.rs:81:30
       |
    81 |         let arena = unsafe { &*(&**arena as *const dyn Send as *const Arena<T>) };
       |                              ^^^^-------^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
       |                                  |
       |                                  backing allocation comes from here
       |
       = note: casting from `dyn Send` (0 bytes) to `Arena<T>` (56 bytes)
       = note: `#[deny(invalid_reference_casting)]` on by default

Author: dtolnay

src/arena.rs

@@ -60,6 +60,7 @@ impl<T> FromIterator<T> for Slice<T>
 where
     T: 'static + Send + Clone,
 {
+    #[allow(invalid_reference_casting)] // false positive: https://github.com/rust-lang/rust/issues/121074
     fn from_iter<I>(iter: I) -> Self
     where
         I: IntoIterator<Item = T>,