Auto merge of rust-lang#133061 - scottmcm:dedup-rc, r=<try>

[DRAFT] `Rc`: allow deduping both `deref` and `clone` across types

This works by *always* putting the counts *just* before the value, rather than using an `RcInner` type at all.

By doing that, the offset to the counts are exactly the same for all types -- small or large, low alignment or high -- so `Rc::clone` can be polymorphized, and `Rc::deref` is always just a no-op.

r? ghost

library/alloc/src/lib.rs

@@ -104,10 +104,12 @@
 #![feature(async_closure)]
 #![feature(async_fn_traits)]
 #![feature(async_iterator)]
+#![feature(box_as_ptr)]
 #![feature(box_uninit_write)]
 #![feature(clone_to_uninit)]
 #![feature(coerce_unsized)]
 #![feature(const_align_of_val)]
+#![feature(const_alloc_layout)]
 #![feature(const_box)]
 #![feature(const_eval_select)]
 #![feature(const_heap)]