feat(rt): add TokioExecutor

[tomkarw]

Hey Sean,
I decided to give hyperium/hyper#2861 a go. I'd like to ramp up the contributions after that.

Not certain what exactly is required, feel free to require additional changes.
I tried to write some basic test, but I don't know of a good way to wait for a spawned task without retaining the handle.

Also, I have a question. Why does the Executor trait require a generic parameter, wouldn't it make sense to only require it on execute function? I.e.:

fn execute<Fut>(&self, fut: Fut)

Closes hyperium/hyper#2861

[davidpdrsn]

Also, I have a question. Why does the Executor trait require a generic parameter, wouldn't it make sense to only require it on execute function? I.e.:

fn execute<Fut>(&self, fut: Fut)

Then you wouldn't be able to make an executor that only works for certain types of futures. Someone might want impl Executor<MyFuture> for MyExecutor

[seanmonstar]

Thanks for the PR! Looks like cargo fmt has its own opinions 🙈

[tomkarw]

Rookie mistake, awkward.

Should be fine now.
However, there is a bunch of clippy warnings in there. Do you mind if I fix them up as part of this PR?

[seanmonstar]

Looks like Miri doesn't like running Tokio in tests, You could add a #[cfg(not(miri)] attribute on it.