StateClass cannot be used for classes that are not created by phper.

[jmjoy]

          In fact, I prefer to change it this way to be consistent with the `implements` method. What do you think?

master...jmjoy:phper-fork:extends

Originally posted by @jmjoy in #190 (comment)

Previously, I mentioned that it was possible to add a StateClass::from_name method, but I realized that I made a critical mistake. The issue is that StateClass creates a StateObject using new_object, and the state held by StateObject is of type *mut dyn Any, which is the fixed state held by phper. However, if a non-phper class also creates a StateObject using new_object, problems will arise—for example, the code below will result in a segmentation fault:

#[php_get_module]
pub fn get_module() -> Module {
    // New `Module` with extension info.
    let mut module = Module::new(
        env!("CARGO_PKG_NAME"),
        env!("CARGO_PKG_VERSION"),
        env!("CARGO_PKG_AUTHORS"),
    );

    module.on_request_init(|| {
        let ex = StateClass::from_name("Exception");
        let ex_obj = ex.new_object([]).unwrap();
        let state = ex_obj.as_state();
    });

    module
}

I need to rethink the design of the ClassEntity::extends method to avoid this issue.