Auto merge of #110353 - the8472:in-place-flatten-chunks, r=cuviper

Expand in-place iteration specialization to Flatten, FlatMap and ArrayChunks

This enables the following cases to collect in-place:

```rust
let v = vec![[0u8; 4]; 1024]
let v: Vec<_> = v.into_iter().flatten().collect();

let v: Vec<Option<NonZeroUsize>> = vec![NonZeroUsize::new(0); 1024];
let v: Vec<_> = v.into_iter().flatten().collect();

let v = vec![u8; 4096];
let v: Vec<_> = v.into_iter().array_chunks::<4>().collect();
```

Especially the nicheful-option-flattening should be useful in real code.

Author: bors