Rollup merge of #74989 - pubfnbar:impl-array-indexing, r=KodrAus

Implement `Index` and `IndexMut` for arrays

Adds implementations of `Index` and `IndexMut` for arrays that simply forward to the slice indexing implementation in order to fix the following problem:

If you implement `Index<MyIndexType>` for an array, you lose all the other indexing functionality that used to be available to the array via its implicit coercion to a slice. An example of what I'm talking about:
```rust
use std::ops::Index;

pub enum MyIndexType {
    _0, _1, _2, _3, _4, _5, _6, _7,
}

impl<T> Index<MyIndexType> for [T; 8] {
    type Output = T;

    fn index(&self, index: MyIndexType) -> &T {
        unsafe { self.get_unchecked(index as usize) }
    }
}

fn main() {
    let array = [11u8; 8];

    println!("{:?}", array[MyIndexType::_0]); // OK

    println!("{:?}", array[0usize]); // error[E0277]
    //               ^^^^^^^^^^^^^ `[u8; 8]` cannot be indexed by `usize`
}
```

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