Add a `teardown` function for GenServer trait

[ElFantasma]

Similarly to init we should provide a teardown function to handle any required duty before tearing down a GenServer. Eg. it can be used to cancel any timer or listener tasks spawned to support the main GenServer.

[juan518munoz]

Wasn't aware of this issue, #22 should handle it, I'll rename the stop funtion to teardown.