[Proposal] new cats.more module

[kailuowang]

Relevant discussion can be found here

I am proposing creating a cats.more module (could be a separate repo) where we can place in non-core, no-so-complex features. Possible candidates for going into this module includes #1935, #1812, #1767, #1755, #1730, #1400, #921, #324
(We shall still decide separately for each of these cases IRT where they should actually go)

It's for stuff that we can’t justify a single module for it and can’t find where it belongs. The API should still be stable to be included in this module though. We may or may not want to provide the same backward compatibility promise with this module as with core

Update: Renamed from "extra" to "more"

[tpolecat]

I'm not convinced this is justifiable for any technical reason, but there may be a benefit in a "restrained" core just to keep the intimidation level down. However if we do this I would like it to be clear that it's not a bunch of random crap, but rather additional features that are less-commonly used. As for naming I kind of like cats-core and cats-more because it doesn't have the junk drawer connotation.

[kailuowang]

I love the idea of naming it cats-more

[LukaJCB]

I'll also refer to #1364, as it also has some type classes that seem like they could go into cats-more :)