Better integration with tokio-console

[loyd]

Related to #171

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

[codecov]

Codecov Report

❌ 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 ⚠️

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[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.