Auto merge of #100023 - saethlin:send-sync-chunksmut, r=m-ou-se

Add back Send and Sync impls on ChunksMut iterators

Fixes rust-lang/rust#100014

These were accidentally removed in #94247 because the representation was changed from `&mut [T]` to `*mut T`, which has `!Send + !Sync`.

Author: bors