Inter-proc channel operations where data is ignored

[grebe]

Consider the situation where a receive is performed and the data is ignored- one scenario where this could happen is a top proc with an array of channels where it receives from each of N identical child procs and for whatever reason the top proc drops the data on the floor for half of the children.

Currently, this results in a few different not-great things:

1. we don't narrow the channels where data is ignored (they could be zero-width)
2. we don't DCE the unread ports¹
3. we don't waive the unread ports

This problem is currently manifesting for me as unread ports (3), but ideally we'd also have (1) and/or (2) to obviate the need for waivers.

Footnotes

1. is it a good thing to DCE ports? It might be useful to keep them around to facilitate testing internal blocks, but I don't think we have a strong contract wrt what internal blocks look like. ↩