Add a variant of slot to ignore child output

[paul-rouse]

The discussion of #636 mentioned the slight inconvenience of replacing const Nothing when child output messages need to be ignored. I had several of these in one of the projects I'm testing against next-6.

How about adding a variant of slot which ignores outputs? It ccould also be used in the Void case, slightly simplifying things there, too. Here's an implementation, but I'm not sure about the name slot_; I wanted to avoid slot' since that meant something different up to version 4.

I realise this makes use of the fact that the Maybe is still there in the underlying implementation, and you may have been intending to remove that sometime in the future, so feel free to ignore this PR if there is a better direction to go!

[garyb]

Seems reasonable to me - and I think the slot_ name makes sense, as underscore-suffixed things often imply some kind of result is being ignored. Thanks!