Verify safety of slice iterator functions (Challenge 18)

Add verification harnesses for all Part 1 and Part 2 functions required
by Challenge 18. This includes:

Part 1 (macros.rs iterator! functions):
- All 16 macro-generated functions (make_slice, len, is_empty, next,
  size_hint, count, nth, advance_by, last, fold, for_each, position,
  rposition, next_back, nth_back, advance_back_by) verified for both
  Iter and IterMut across 4 representative types ((), u8, char, (char,u8))

Part 2 (iter.rs):
- 9 __iterator_get_unchecked contracts verified (Windows, Chunks,
  ChunksMut, ChunksExact, ChunksExactMut, RChunks, RChunksMut,
  RChunksExact, RChunksExactMut)
- 33 safe functions with unsafe code verified (Iter::new, IterMut::new/
  into_slice/as_mut_slice, Split::next/next_back, all Chunks/ChunksExact/
  RChunks/RChunksExact variants, ArrayWindows next/nth/next_back/nth_back)

Uses #[cfg(kani)] abstractions in macros.rs for fold/for_each/position/
rposition to enable unbounded verification of loop-based functions.

Author: jrey8343