Unstable book `lang_items` example is outdated

[cyrgani]

Location

https://github.com/rust-lang/rust/blob/master/src/doc/unstable-book/src/language-features/lang-items.md

Summary

The example uses #[rustc_box], which was removed in #135046. Unfortunately, the new intrinsic is at alloc::boxed::box_new and adding extern crate alloc to use it causes the real Box to conflict with the custom Box.

@rustbot label A-lang-item

[Noratrieb]

This page should probably be deleted from the unstable book and added to the rustc dev guide