Add Doc.lineOr

[johnynek]

close #120 cc @DavidGregory084

This addresses some of the issues raised #76 and #117

Namely, I took the same strategy as #117, but do not expose Line at the top level. This allows us to maintain all the invariants we have, but does not give us the feature request of #76

By changing this PR only to remove the sys.error in flatten on Line and allowing Line at the top level, the following laws fail:

- fill matches spec
- a is flat ==> Concat(a, Union(b, c)) === Union(Concat(a, b), Concat(a, c))
- a.flatten == a.flattenOption.getOrElse(a)
- group law
    /**
     * group(x) = (x' | x) where x' is flatten(x)
     *
     * (a | b)*c == (a*c | b*c) so, if flatten(c) == c we have:
     * c * (a | b) == (a*c | b*c)
     *
     * b.grouped + flatten(c) == (b + flatten(c)).grouped
     * flatten(c) + b.grouped == (flatten(c) + b).grouped
     */

Note this is minorly binary incompatible. We will need to bump the minor version here, but it should be source compatible.

Lastly, I want to apologize to anyone who is disappointed at the pace of change here. This code is super highly tested with very strong law coverage, but that also constrains the implementation quite a bit. It seems like we have to give up some of the laws to get a hardLine (feature requested from #76). @seanmcl did some excellent work trying to get this in, and I lost track of it and he possibly lost interest. In the mean time, @olafurpg also started using a fork due to the lack of this feature. It's a bummer. If I had been more engaged we would be further along in this repo.

What I propose is merge this, which is part of @seanmcl 's work, and then we can see explore the minimal set of laws we can lose to expose Line at the top level.

I think flattening and fill seem to be the ones that are the most at risk.

[codecov-io]

Codecov Report

Merging #176 into master will decrease coverage by 0.33%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #176      +/-   ##
==========================================
- Coverage   98.18%   97.84%   -0.34%     
==========================================
  Files           4        4              
  Lines         275      279       +4     
  Branches       19       27       +8     
==========================================
+ Hits          270      273       +3     
- Misses          5        6       +1

Impacted Files	Coverage Δ
...re/src/main/scala/org/typelevel/paiges/Chunk.scala	98.24% <100%> (+0.03%)	⬆️
core/src/main/scala/org/typelevel/paiges/Doc.scala	98% <100%> (-0.48%)	⬇️

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[non]

I'm also sorry that I didn't prioritize this more highly. I think your plan makes sense -- hopefully we can figure something out.

👍 on this PR