Change ConditionallySelectable supertrait

[tarcieri]

To resolve #94, removes the Copy supertrait bound on ConditionallySelectable, replacing it with Sized instead.

It turns out the bound is only used in the default implementation of ConditionallySelectable::conditional_swap, and is easy to replace by slightly changing that default implementation.

Removing this supertrait bound is arguably a breaking change since it means types which impl ConditionallySelectable can no longer be assumed to be Copy, so also bumps the version to 3.0.0-pre.

Alternative to #118, #136

[AaronFeickert]

Not sure if reviews are welcome from folks without merge authority, but LGTM aside from the MSRV-related CI failure.

[tarcieri]

This would be more flexible if it were Clone instead, and that wouldn't impact performance, but I guess there are worries that Clone impls won't run in constant time

[cmlsharp]

I'm not sure why Copy is necessary here?

fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self {
    core::array::from_fn(|i| T::ct_select(&a[i], &b[i], choice))
}

If you want to eliminate the bounds checks (which is sound since a, b, and the return value all statically have length N) you can do

fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self {
    // # SAFETY: a, b, and return value all have length exactly N
    // the N annotation isn't necessary here, just being explicit
    core::array::from_fn::<_, N, _>(|i| {
        T::ct_select(
            unsafe { a.get_unchecked(i) },
            unsafe { b.get_unchecked(i) },
            choice,
        )
    })

}