Add tokio02 feature flag

[yoshuawuyts]

Adds the tokio02 feature flag. This forwards to smol's tokio02 feature flag which runs tokio::rt::enter whenever a new thread is created. This should allow running async_std::main and having tokio crates just work ™. Thanks!

[skade]

This transitively ties us to the currently internal runtime and requests it to have tokio support. I'm not super sure what would be a remedy for that, but we should make clear what the expectations from our side top expose such a feature are.

[yoshuawuyts]

This transitively ties us to the currently internal runtime and requests it to have tokio support.

Adding this feature isn't too difficult even if we were to switch internal engines -- the way this works is calling tokio::rt::enter every time we call thread::spawn. This feature is really useful for projects currently using tokio but wanting to switch away from it. It enables switching over one dependency at the time rather than being forced into a "rewrite the world" scenario.

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@skade could you maybe say more what you mean by: "[...] the expectations from our side top expose such a feature are."

[skade]

I was very tired, so sentence makes no sense.

What I wanted to say is that the feature should be specified from the top down instead of bottom up.