cpu_control::start_app_core improvements

[dimpolo]

• add Send bound
• move #[must_use] to AppCoreGuard
• remove redundant -> ()
• document guard behaviour

The added Send bound is a breaking change but required for soundness.

[dimpolo]

Also, @bjoernQ is there a reason two lifetimes are involved in:

pub fn start_app_core<'a: 'b, 'b>(
        &mut self,
        entry: &'a mut (dyn FnMut() -> () + 'a),
    ) -> Result<AppCoreGuard<'b>, Error> {
    ...
}

It isn't clear to me so I'd like to document it :)

[bjoernQ]

It seems the lifetime 'b isn't really needed anymore. I think I added it at some point during development and forgot about it so you could remove it

[dimpolo]

Done

[bjoernQ]

LGTM - thanks a lot for this PR!