Move FunctorEmpty and TraverseEmpty back into cats-core?

[LukaJCB]

FunctorEmpty and TraverseEmpty (used to be FunctorFilter and TraverseFilter were moved to cats-mtl when that was extracted from cats-core, as it was one of multiple leaves for Functor.
However, when I look at these two type classes, they seem to be awfully unfit for cats-mtl, as they don't have anything at all to do with monad transformers and can't even be lifted through monad stacks. I suspect the only reason they were moved at all was to use the new composition-based type class encoding (which I agree is a good reason).

I propose moving them back along with their instances as they are really useful and we shouldn't have any trouble using the composition-based type class encoding (we already use it for things like Parallel). Interested in hearing thoughts :)

[ceedubs]

I'm a bit partial to these type classes being in core, but I'm also the person who originally added them, so I'm a bit biased.

Note that this is also somewhat relevant to #1755.

I think that there are others who are more qualified than I am to weigh-in on the composition-based type class encoding component.

[kailuowang]

I suspect the only reason they were moved at all was to use the new composition-based type class encoding

As far as I recall that was the main drive, although there might've been some technical reasons as well. I believe @edmundnoble also mentioned some kinkiness in writing laws for them. But I don't recall the detail and from the law code in cats-mtl, I didn't immediately see any technical issues (if to move them to core.) I'd say worth a try.

I missed having these classes in core a couple of times. so I'm also slightly +1 on moving them back too.