Auto merge of #101837 - scottmcm:box-array-from-vec, r=m-ou-se

Add `Box<[T; N]>: TryFrom<Vec<T>>`

We have `[T; N]: TryFrom<Vec<T>>` (#76310) and `Box<[T; N]>: TryFrom<Box<[T]>>`, but not this combination.

`vec.into_boxed_slice().try_into()` isn't quite a replacement for this, as that'll reallocate unnecessarily in the error case.

**Insta-stable, so needs an FCP**

(I tried to make this work with `, A`, but that's disallowed because of `#[fundamental]` rust-lang/rust#29635 (comment))

Author: bors