Add global state management

[Totodore]

This PR adds global state management as well as a State extractor to get the state in any handlers.

Note : Before merging this PR, it is necessary to use dynamics handler for the on_disconnect function. Otherwise it wont be possible to use the state in these handlers.

Currently it is not possible to use the references in extractors because of the following problem :
In the ConnectHandler implementation below for any function with arguments that implements FromConnectParts and are specified by values. To add a lifetime to extractors we could add a lifetime on the trait FromConnectParts. The problem is that the provided lifetime should be the one declared with the for<'a> corresponding to the function and its scope is rather limited.

impl<A, F, $($ty,)*> ConnectHandler<A, (private::Sync, $($ty,)*)> for F
        where
            F: for<'a> FnOnce($($ty,)*) + Send + Sync + Clone + 'static,  // The lifetime 'a should be propagated to the `FromConnectParts` 
            A: Adapter,                                                   // ---------------------------------------
            $( $ty: FromConnectParts<'a, A> + Send, )*                    // <----- here
        {
            fn call(
                &self,
                s: Arc<Socket<A>>,
                auth: Option<String>,
                state: &Arc<dyn std::any::Any + Send + Sync>)
            {
                $(
                    let $ty = match $ty::from_connect_parts(&s, &auth,  state) {
                        Ok(v) => v,
                        Err(_e) => {
                            #[cfg(feature = "tracing")]
                            tracing::error!("Error while extracting data: {}", _e);
                            return;
                        },
                    };
                )*

                (self.clone())($($ty,)*);
            }
        }

Because of this issue any given state must be clonable so it can be given by value to the handlers.

Close :

• State Management #178

[tausifcreates]

States are cheap to clone right? So this should be fine. 😄

[Totodore]

No, it depends if the state is put in an Arc or not.
State structs are provided by the user and could be any size. Moreover they need to be Sync so that the state is shared and not copied.

[tausifcreates]

Oh, great! I assumed you were going to implement clone-able state because of the said limitations 😄

[github-advanced-security]

clippy found more than 20 potential problems in the proposed changes. Check the Files changed tab for more details.

[Totodore]

Superseded by #194