Make Engine Predicate -> Kernel Expression Visitor consistent with Kernel -> Engine visitor

[OussamaSaoudi]

Please describe why this is necessary.

The model for Engine -> kernel expression visiting is different from Kernel -> Engine visitor.

In particular, visit_expression_and is not great to implement over ffi due to the EngineIterator. It would be easier to use the child list abstraction that schema visitor and kernel->engine visitor use.

Moreover, it is not intuitive that the *const c_void child is casted to a usize in this:

#[no_mangle]
pub extern "C" fn visit_expression_and(
    state: &mut KernelExpressionVisitorState,
    children: &mut EngineIterator,
) -> usize {
    let result = Expression::and_from(
        children.flat_map(|child| unwrap_kernel_expression(state, child as usize)),
    );
    wrap_expression(state, result)
}

Describe the functionality you are proposing.

No response

Additional context

No response