implement `arbitrary::Arbitrary` for `SmallVec`

[alejandro-vaz]

implement arbitrary::Arbitrary for SmallVec

there's an implementation already on the v1 branch, revise it and port it to v2

use {
    crate::{Array, SmallVec},
    arbitrary::{Arbitrary, Unstructured},
};

impl<'a, A: Array> Arbitrary<'a> for SmallVec<A>
where
    <A as Array>::Item: Arbitrary<'a>,
{
    fn arbitrary(u: &mut Unstructured<'a>) -> arbitrary::Result<Self> {
        u.arbitrary_iter()?.collect()
    }

    fn arbitrary_take_rest(u: Unstructured<'a>) -> arbitrary::Result<Self> {
        u.arbitrary_take_rest_iter()?.collect()
    }

    fn size_hint(depth: usize) -> (usize, Option<usize>) {
        arbitrary::size_hint::and(<usize as Arbitrary>::size_hint(depth), (0, None))
    }
}

it should be locked behind a cfg "arbitrary", but inside the lib.rs file