ipc-channel-mux router support

The IPC channel multiplexing crate, ipc-channel-mux, now includes a “router”. The router provides a means of automatically forwarding messages from subreceivers to Crossbeam receivers so that users can enjoy Crossbeam receiver features, such as selection (explained below). The absence of a router blocked the adoption of the crate by Servo, so it was an important feature to support. Routing…