feat: Closed error code

[dzmitry-lahoda]

When I close connection on tokio server I provide next:

pub struct CloseFrame {
    /// The reason as a code.
    pub code: u16,
    /// The reason as text string.
    pub reason: Utf8Bytes,
}

When I receive message using ewebsock I get next:

Closed

It is possible to have Closed(Option<u16>)? Or there is some known limitation of RFC or consistency of impls?

[dzmitry-lahoda]

I can PR this if in general it is viable and possibly acceptable.

[emilk]

A PR would be nice!