Rollup merge of #71421 - elichai:2020-04-boxed-slice, r=sfackler

Add a function to turn Box<T> into Box<[T]>

Hi,
I think this is very useful, as currently it's not possible in safe rust to do this without re-allocating.
an alternative implementation of the same function can be:
```rust
pub fn into_boxed_slice<T>(boxed: Box<T>) -> Box<[T]> {
    unsafe {
        let slice = slice::from_raw_parts_mut(Box::into_raw(boxed), 1);
        Box::from_raw(slice)
    }
}
```

The only thing that makes me a little uncomfortable is this line :
> The alignment of array types is greater or equal to the alignment of its element type

from https://rust-lang.github.io/unsafe-code-guidelines/layout/arrays-and-slices.html

But then I see:
> The alignment of &T, &mut T, *const T and *mut T are the same, and are at least the word size.
> The alignment of &[T] is the word size.

from https://rust-lang.github.io/unsafe-code-guidelines/layout/pointers.html#representation

So I do believe this is valid(FWIW it also passes in miri https://play.rust-lang.org/?gist=c002b99364ee6b29862aeb3565a91c19)

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