Auto merge of rust-lang#52206 - RalfJung:zst-slices, r=alexcrichton

slices: fix ZST slice iterators making up pointers; debug_assert alignment in from_raw_parts

This fixes the problem that we are fabricating pointers out of thin air. I also managed to share more code between the mutable and shared iterators, while reducing the amount of macros.

I am not sure how useful it really is to add a `debug_assert!` in libcore. Everybody gets a release version of that anyway, right? Is there at least a CI job that runs the test suite with a debug version?

Fixes rust-lang#42789

Author: bors