remove unnecessary Option for mio::Events in Driver

[cjwcommuny]

Motivation

tokio::runtime::io::Driver has a field events with the typeOption<mio::Events>. The Option wrapper is used to solve the second mutable borrow conflicts in the method Driver::turn. This requires to take the inner mio::Events every time Driver::turn is called, which introduce unnecessary overheads.

Solution

1. Change Option<mio::Events> to mio::Events so that no Option::take is needed to call in the method Driver::turn.

2. Change the function signature of Driver::dispatch from:

   fn dispatch(&mut self, token: mio::Token, ready: Ready)

   to

   fn dispatch(resources: &mut Slab<ScheduledIo>, tick: u8, token: mio::Token, ready: Ready)

   so that the compiler won't complain.

[Darksonn]

This seems good, thanks.

[Darksonn]

The CI failures appear to be some sort of outage in GitHub's caching setup. status page

[cjwcommuny]

I reformat the code.