Make callbacks Option<dyn FnMut>

[cmleinz]

We can relax the trait bounds for callbacks to be FnMut rather than Fn, also can remove the Sync bound

[craftytrickster]

Removing sync probably isn't a big deal, but changing the types on the options is a breaking change.

If you are to do that, we should bump the minor version of the crate, and, at the same time, take advantage to implement - #23

[cmleinz]

Sounds good to me. If the on_disconnect callback gets an arg with the error what should the type be? Since we initiate the callback on Ok(0) as well as Err(err).

[craftytrickster]

After giving this another look, I am not entirely convinced it is worth adding additional information to the trying to connect message. What do you think?

on_connection failed probably should have Err and you could argue that on_disconnection could have an Optional Err.

As a user, do you think having those extra errors are actually valuable?

[craftytrickster]

I think for now, it isn't worth changing the interface, I prefer to close this.