`Vector::new_with_bits` accepts values that make the very first `push_back` panic

[DRMacIver]

fn main() {
    let v: rpds::Vector<i32> = rpds::Vector::new_with_bits(64);
    let v = v.push_back(0);
    println!("{:?}", v.get(0));
}

thread 'main' panicked at src/vector/mod.rs:387:9:
attempt to shift left with overflow

new_with_bits only requires bits > 0, so 64 is accepted and construction succeeds; the first push_back then panics in debug (in release it does not panic). Either new_with_bits should reject a bits value it can't support, or the resulting Vector should be usable — not accept it and then panic on the first push.

Tested on rpds 1.2.1.

BTW, this bug was found using hegel. Happy to contribute the tests if you're interested.