I'm trying to implement iterated SHA256. This works:

```
use sha2::{Digest, Sha256}; // 0.8.2

fn main() {
    let preimage = [42; 80];
    let hash = Sha256::digest(&Sha256::digest(&preimage));
}

```

This doesn't:

```
use sha2::{Digest, Sha256}; // 0.8.2

fn main() {
    let preimage = [42; 80];
    let mut hash = preimage;
    for _ in 0..2 {
        hash = Sha256::digest(&hash);
    }
}

```

I get an error:

```none
error[E0308]: mismatched types
 --> src/main.rs:7:16
  |
7 |         hash = Sha256::digest(&hash);
  |                ^^^^^^^^^^^^^^^^^^^^^ expected array `[u8; 80]`, found struct `generic_array::GenericArray`
  |
  = note: expected array `[u8; 80]`
            found struct `generic_array::GenericArray<u8, typenum::uint::UInt<typenum::uint::UInt<typenum::uint::UInt<typenum::uint::UInt<typenum::uint::UInt<typenum::uint::UInt<typenum::uint::UTerm, typenum::bit::B1>, typenum::bit::B0>, typenum::bit::B0>, typenum::bit::B0>, typenum::bit::B0>, typenum::bit::B0>>`

```

I would like the second style so I can easily re-iterate more than twice.