Build mandelbrot-iter on CI & use cabal-docspec

[phadej]

There are plenty of prop> in free, but most of them aren't "decidable", e.g.

src/Control/Monad/Free/Church.hs:--   prop> fmap f . fmap g == fmap (f . g)
src/Control/Monad/Trans/Iter.hs:-- prop> runIter . iter == id

(or/and would require quite a large setup to ctually run, QuickCheck instances etc.).

---

With this PR I'm able to run cabal-docspec on a single kmettverse project at once. That is quite cool.

[RyanGlScott]

I recall that I originally disabled mandelbrot-iter due to xpika/hgl#3, but fortunately, it seems that a fix for that issue has made its way into HGL-3.2.3.2 on Hackage. In light of this, can you remove the following comments from free-examples?

free/examples/free-examples.cabal

Lines 46 to 48 in b6ce7a2

	-- This example depends on the HGL library which, unfortunately, fails to
	-- compile on GHC 8.4 or later. (See https://github.com/xpika/hgl/issues/3.)
	-- Build at your own risk.

[RyanGlScott]

Thanks for all your work on this, @phadej! If you apply my suggestion in #206 (review), then I'll merge this.

[phadej]

I changed the mandelbrot-iter example: removed the comment and added lowerbound on HGL