Use bolero generators from any location in kani proof

[camshaft]

Currently, bolero generators require a Driver to generate a value. This requires some set up and implementing quite a few trait methods. For kani proofs, generating arbitrary values can potentially happen in any location. Ideally, we would include a method on the ValueGenerator trait that would allow kani proofs to leverage bolero generators.

#[derive(Debug, TypeGenerator)]
struct MyStruct {
    #[generator(0..4)]
    my_field: u64
}

#[kani::proof]
fn my_proof() {
    let v = gen::<MyStruct>().any();
    assert!((0..4).contains(&v.my_field));
}