This is my first PR; be gentle!

In https://users.rust-lang.org/t/why-does-vec-from-raw-parts-require-same-size-and-not-same-size-capacity/73036/2?u=janpaul123 it was suggested to me that I should make a PR to make the documentation of `Vec::from_raw_parts` less strict, since we don't require `T` to have the same size, just `size_of::<T>() * capacity` to be the same, since that is what results in `Layout::size` being the same in `dealloc`, which is really what matters.

Also in https://users.rust-lang.org/t/why-does-vec-from-raw-parts-require-same-size-and-not-same-size-capacity/73036/8?u=janpaul123 it was suggested that it's better to use `slice::from_raw_parts`, which I think is useful advise that could also be mentioned in the docs, so I added that too.

Let me know what you think! :)

Clarify that ManuallyDrop<T> has same layout as T

This PR implements the documentation change under discussion in rust-lang/unsafe-code-guidelines#302. It should not be approved or merged until the discussion there is resolved.

…s-with-different-allocators, r=dtolnay

Allow comparing `Vec`s with different allocators using `==`

See https://stackoverflow.com/q/71021633/7884305.

I did not changed the `PartialOrd` impl too because it was not generic already (didn't support `Vec<T> <=> Vec<U> where T: PartialOrd<U>`).

Does it needs tests?

I don't think this will hurt type inference much because the default allocator is usually not inferred (`new()` specifies it directly, and even with other allocators, you pass the allocator to `new_in()` so the compiler usually knows the type).

I think this requires FCP since the impls are already stable.

Docs: make Vec::from_raw_parts documentation less strict

This is my first PR; be gentle!

In https://users.rust-lang.org/t/why-does-vec-from-raw-parts-require-same-size-and-not-same-size-capacity/73036/2?u=janpaul123 it was suggested to me that I should make a PR to make the documentation of `Vec::from_raw_parts` less strict, since we don't require `T` to have the same size, just `size_of::<T>() * capacity` to be the same, since that is what results in `Layout::size` being the same in `dealloc`, which is really what matters.

Also in https://users.rust-lang.org/t/why-does-vec-from-raw-parts-require-same-size-and-not-same-size-capacity/73036/8?u=janpaul123 it was suggested that it's better to use `slice::from_raw_parts`, which I think is useful advise that could also be mentioned in the docs, so I added that too.

Let me know what you think! :)

…olnay

impl From<&[T; N]> and From<&mut [T; N]> for Vec<T>

I really wanted to write:

```rust
fn example(a: impl Into<Vec<u8>>) {}

fn main() {
    example(b"raw");
}
```