Clean-up Network.Mux.Channel

[coot]

We have Network.Mux.Channel and Ouroboros.Network.Channel which do not differ much. Network.Mux.Channel.Channel m = Ouroboros.Network.Channel.Channel m ByteString. My short experiment suggests that:

• we can remove all auxiliary functions from Network.Mux.Channel they are not
  used by network-mux package at all.
• there's just one place where one of the Network.Mux.Channel functions is
  used in Ouroboros.Network.Channel and it can be replaced
• we could either move the network-mux Channel entirely to
  Network.Mux.Types or make it an internal module. We should instead expose
  the type from Network.Mux or Network.Mux.Types as it's part of essential
  public API.

Furthermore we should consider to move the auxiliary functions of
Ouroboros.Network.Channel to ouroboros-network-testing: I suspect they are
only used in various demos & tests.