fix allocator API `into_boxed_slice` and `into_vec`

[alejandro-vaz]

    #[inline]
    pub fn into_boxed_slice(self) -> Box<[T]> {
        self.into_vec().into_boxed_slice()
    }

and also

pub fn into_vec(self) -> Vec<T> {

Both are UBs.

Whoops, I thought I left all the stdlib-interacting functions only implemented for SmallVec<_, Global> (from_vec is correct).

Originally posted by @bolshoytoster in #630 (comment)

[alejandro-vaz]

and is there any special reason there's no From<SmallVec<T, N, A>> for Vec<T, A> and so on?? to me it looks like missing APIs

[bolshoytoster]

and is there any special reason there's no From<SmallVec<T, N, A>> for Vec<T, A> and so on?? to me it looks like missing APIs

That won't work when using allocator-api2, they'd be using different Allocator traits.

[alejandro-vaz]

no, allocator_api2::vec::Vec solves it

but yeah it's a different Vec, not an interoperability, and I don't think it should be