Fix v8::OwnedIsolate memory leak caused by retain cycle in MiniV8::create_function

[AlaaAlHallaq]

The only way to test this at least for my current expertise in Rust :

• is by adding a print statement in drop impl for OwnedIsolate or a break point.
  the following example:

    let ctx = MiniV8::new();
    let obj = ctx.create_object();
    let fc = ctx.create_function(|env| {
        let me = env
            .this
            .as_object()
            .ok_or(mini_v8::Error::FromJsConversionError {
                from: "value",
                to: "object",
            })?;
        let v: Value = me.get("name")?;
        return Ok(v);
    });
    obj.set("name", "jack").unwrap();
    obj.set("getName", fc).unwrap();
    ctx.global().set("person", obj).unwrap();
    let returned_name: String = ctx.eval("person.getName()").unwrap();
    println!("person.getName() = {:?}", returned_name);
    drop(ctx);

before this apply this pr nether the print statement nor the break point will be reached & v8::OwnedIsolate wont be dropped.
after applying it, it just works.