PR: #40943

Adds an offset_to method to calculate the distance between two raw pointers.

List o' stuff:

• Rename to offset_from and make unsafe (with wrapping_offset_from for the safe one) Tracking issue for ptr::offset_from (feature: ptr_offset_from) #41079 (comment) -- done in Introduce unsafe offset_from on pointers #49297
  • Should offset_from abort instead of panic on ZSTs?
• Given that the difference between arbitrary pointers is meaningless and subject to change at the whims of LLVM anyway, do we even want the safe version? Tracking issue for ptr::offset_from (feature: ptr_offset_from) #41079 (comment) Experience reports from the field appreciated.
• Should there be more methods like this with different restrictions (to allow nuw/nsw, perhaps) or that return usize (like how isize-taking offset is more conveniently done with usize-taking add these days)
• Ensure that our safety requirements align with the proposed llvm.psub [LLVM9+] Use llvm.psub in offset_from #57818