Find current runtime

[nihohit]

Is your feature request related to a problem? Please describe.
I would like to know whether there's a runtime currently set, or whether a spawn/block_on will cause a runtime to be created. Additionally, if a runtime is present, I'd like to know if it's shutting down or not.

Describe the solution you'd like
a static function that returns an Option<RuntimeInfo>, where RuntimeInfo contains pertinent information, and is None if no runtime is set.