feat: make Executor trait object safe

[kyle-mccarthy]

I moved the Sized bound from the Executor trait to the two initializer methods. This makes the trait object safe, allowing for the following

struct Context {
    engine: Arc<dyn Executor>
}

[chenwanqq]

I think this change didn't work? I found it's still not object safe.
struct SimpleWriteCode {
executor: Box,
}
and this Executor shows a 'cannot be made into an object' error