Rollup merge of #110635 - scottmcm:zst-checks, r=the8472

More `IS_ZST` in `library`

I noticed that `post_inc_start` and `pre_dec_end` were doing this check in different ways

d19b64fb54/library/core/src/slice/iter/macros.rs (L76-L93)

so started making this PR, then added a few more I found since I was already making changes anyway.

library/core/src/slice/iter/macros.rs

@@ -73,7 +73,7 @@ macro_rules! iterator {
             // Unsafe because the offset must not exceed `self.len()`.
             #[inline(always)]
             unsafe fn post_inc_start(&mut self, offset: usize) -> * $raw_mut T {
-                if mem::size_of::<T>() == 0 {
+                if T::IS_ZST {
                     zst_shrink!(self, offset);
                     self.ptr.as_ptr()
                 } else {