Unroll factor in `#[unroll]` attributes should accept any const expr, not just literals

Related to the #![feature(loop_hints)] attributes: https://github.com/rust-lang/rust/issues/156874

It would be nice if the N in #[unroll(N)] could take an arbitrary const expression, not just an integer literal:

fn foo<const UNROLL_FACTOR: usize>() {
    #[unroll(UNROLL_FACTOR)]
    loop {
        ...
    }
}

If that's not possible for technical reasons, another possibility would be a std::hint::unroll function which takes a const parameter and a closure:

fn foo<const UNROLL_FACTOR: usize>() {
    std::hint::unroll::<UNROLL_FACTOR>(|| loop {
        ...
    });
}

This would make it easier to experiment with different unroll factors without having to duplicate the same function for each unroll factor

Start by reading the related #![feature(loop_hints)] issue, #156874, and the current #[unroll] attribute behavior. Determine whether arbitrary const expressions can be accepted or whether the proposed std::hint::unroll alternative is needed. Done means const-generic unroll factors work without duplicating functions, with coverage for the supported form.