Separate free package into its own module

[ceedubs]

It's quite possible that people are going to throw tomatoes at me for
proposing this change. There has been much discussion about
modularization in the past, and in fact free used to have its own
module, which was removed in #760/#765.

One significant change that has happened since then is that State has
started to use Eval instead of Trampoline for trampolining. At this
point, nothing outside of the free package in cats-core depends on
free (which is why this PR was so easy to create). To me that makes it
a fairly convenient border for modularization.

There was a brief discussion about this on Gitter in which @sellout supported this change on the basis that they prefer to use fixpoint types (such as Mu) for free structures. Putting free into a separate module may be better for people who want to use different variations of these structures.

Of course in addition to these more recent updates, there are the usual
arguments for modularization and decreased jar size (which I believe is
especially important for catsJS users).

[ceedubs]

I'm particularly interested in feedback from @mpilquist, @travisbrown, @stew, @refried, and @milessabin, as they had previously approved of pulling free into core (but a bit has changed since then as mentioned in the description above).

[milessabin]

I think that Eval taking over from Trampoline makes a world of difference. This gets a 👍 from me.

[ceedubs]

Another item that I meant to mention is #845. Basically there are many more free structures that could potentially be added, and if we choose to go down that path we might not want to add too much "bloat" to core.

[codecov-io]

Current coverage is 81.99%

Merging #990 into master will not change coverage

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  Lines          2704       2704          
  Methods        2639       2639          
  Messages          0          0          
  Branches         60         60          
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  Hits           2217       2217          
  Misses          487        487          
  Partials          0          0          

Powered by Codecov. Last updated by 8355a19...c11ddcf

[julien-truffaut]

What about moving free to it's own project? it seems there is quite a lot of development in this area and it might be easier to experiment without being locked with cats releases.

[adelbertc]

👍 from me

[adelbertc]

Woah there -2.61%

[mpilquist]

👍, either for a separate module or even a separate repo.

[ceedubs]

@adelbertc validateJVM wasn't running the free tests. I've fixed it, and codecov now shows no difference in coverage (though it didn't post an updated comment).

I think I'm going to add some notes in the relevant documentation about this being in a separate module and then merge this. I'll post a follow-up issue about whether it should be in its own project, but I don't want that indecision to keep it in core when it sounds like people are agreed that it shouldn't be there.