Better integration with `tokio-console`

[loyd]

Related to https://github.com/elfo-rs/elfo/issues/171

tokio-console for the usage example compiled with the --cfg tokio_unstable:

[codecov[bot]]

Codecov Report

:x: Patch coverage is 86.36364% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
elfo-core/src/task.rs	84.21%	6 Missing :warning:

:loudspeaker: Thoughts on this report? Let us know!

[loyd]

The console warns about "tasks have been boxed by the runtime due to their size" due to https://github.com/tokio-rs/tokio/blob/637fc1d103882dd04ce434d1fed4a0026fc7b6c2/tokio/src/runtime/mod.rs#L399 I don't totally agree that it's a warning, but we can avoid it by boxing the actors in the supervisor.