Rollup merge of #59328 - koalatux:iter-nth-back, r=scottmcm

Implement specialized nth_back() for Box and Windows.

Hi there, this is my first pull request to rust :-)

I started implementing some specializations for DoubleEndedIterator::nth_back() and these are the first two. The problem has been discussed in #54054 and nth_back() is tracked in #56995.

I'm stuck with the next implementation so I though I do a PR for the ones I'm confident with to get some feedback.

diff --cc src/liballoc/boxed.rs
Simple merge

diff --cc src/liballoc/lib.rs
index 440ce8ac5e842d47ad5ee631f4ecdfa56bbb6c40,da1afc2ff8aa3733df03ee287057954b2eb1afe7..90ff56814fbb153b97dd4452dd53aee29bccfc27
--- 1/src/liballoc/lib.rs
--- 2/src/liballoc/lib.rs
+++ b/src/liballoc/lib.rs
@@@ -112,9 -112,10 +112,10 @@@
  #![feature(rustc_const_unstable)]
  #![feature(const_vec_new)]
  #![feature(slice_partition_dedup)]
 -#![feature(maybe_uninit)]
 +#![feature(maybe_uninit, maybe_uninit_slice, maybe_uninit_array)]
  #![feature(alloc_layout_extra)]
  #![feature(try_trait)]
+ #![feature(iter_nth_back)]
  
  // Allow testing this library
  

diff --cc src/libcore/slice/mod.rs
Simple merge

diff --cc src/libcore/tests/slice.rs
Simple merge