[granc-core] Reduce dependencies on Tokio

[JasterV]

Granc Core uses Tokio both to construct streams and for mpsc channels.

We could either use the sync version of mpsc or try out a more lightweight solution.

The idea behind this is to reduce compile times and also I find it too much to depend on such a huge library as Tokio only for these minimal features.

Tokio as a dev-dependency instead is ok.