Challenge 18: Verify safety of Slice iterator functions

Add Kani proof harnesses for slice iterator functions specified in
Challenge #18, covering safe abstractions and unsafe
__iterator_get_unchecked implementations for Chunks, ChunksMut,
ChunksExact, ChunksExactMut, Windows, ArrayWindows, RChunks,
RChunksMut, RChunksExact, RChunksExactMut, Split, Iter, and IterMut.
Resolves #282

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Samuelsills