Rollup merge of #62959 - LukasKalbertodt:array-value-iter, r=scottmcm

Add by-value iterator for arrays

This adds an iterator that can iterate over arrays by value, yielding all elements by value. However, **this PR does _not_ add a corresponding `IntoIterator` impl for arrays**. The `IntoIterator` impl needs some discussion about backwards-compatibility that should take place in a separate PR. With this patch, this code should work (but there is currently still a bug):

```rust
#![feature(array_value_iter)]
use std::array::IntoIter;

let arr = [1, 2, 3];
for x in IntoIter::new(arr) {
    println!("{}", x);
}
```

**TODO**:
- [x] Get initial feedback
- [x] Add tests
- [x] Figure out why stage1 produces weird bugs ([comment](https://github.com/rust-lang/rust/pull/62959#issuecomment-516016524))
- [x] Add UI tests as mentioned [here](https://github.com/rust-lang/rust/pull/62959#discussion_r307061894) (will do that soon-ish)
- [x] Fix [this new bug](https://github.com/rust-lang/rust/pull/62959#issuecomment-544732159)

**Notes for reviewers**
- Is the use of `MaybeUninit` correct here? I think it has to be used due to the `Clone` impl which has to fill the dead array elements with something, but cannot fill it with a correct instance.
- Are the unit tests sufficient?

CC #25725

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