Rollup merge of #80600 - CoffeeBlend:maybe_uninit_array_assume_init, r=dtolnay

Add `MaybeUninit` method `array_assume_init`

When initialising an array element-by-element, the conversion to the initialised array is done through `mem::transmute`, which is both ugly and does not work with const generics (see #61956). This PR proposes the associated method `array_assume_init`, matching the style of `slice_assume_init_*`:

```rust
unsafe fn array_assume_init<T, const N: usize>(array: [MaybeUninit<T>; N]) -> [T; N];
```

Example:
```rust
let mut array: [MaybeUninit<i32>; 3] = MaybeUninit::uninit_array();
array[0].write(0);
array[1].write(1);
array[2].write(2);

// SAFETY: Now safe as we initialised all elements
let array: [i32; 3] = unsafe {
     MaybeUninit::array_assume_init(array)
};
```

Things I'm unsure about:
* Should this be a method of array instead?
* Should the function be const?

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