Named sends and receives for IOConstraints

Codegen supports scheduling constraints between (channel_name, send/recv) pairs. If there is only one send or receive to a channel (which is currently a requirement for codegen), this makes sense. However, for II>1, you could have multiple sends and receives to the same channel and want to independently constraint them. There should be a mechanism to name send/recvs (or tokens?) and introduce scheduling constraints between the named entities.

Example: a proc is talking to a memory with `request`, `read_response`, and `write_completion` channels. You want to have a constraint between a write request and write completion, and a separate constraint between a read request and read response, but both requests happen on the same channel so you end up overconstrained (and, if the requests aren't mutually exclusive, potentially infeasible).