Add more standard rules and eliminate Seq views

[Jacoby6000]

In this PR, I've created strict variants of all the base property rules, and I've removed the Seq view restrictions that were on some of them... Instead, they now depend on either Foldable, Sizeable, Monoid, or Indexable.

• Sizeable is just a typeclass for things which have size.
• Indexable was updated to be based on mapWithIndex rather than zipWithIndex. This lets us generalize Indexable, such that forall F[_]: Foldable, F is also Indexable. Additionally, Indexable will be able to be removed once we move to cats 1.0.0, since these things are built in to Traverse there.
• Monoid is used for the non-empty rule, rather than the seq view that was there before. This means users can define their own things which may be empty.

Some consequences of this, are that Seq won't work for anything which depends on Foldable. Depending on cats' alleycats package should get you unlawful typeclasses for those, however.

[codecov]

Codecov Report

Merging #9 into develop will increase coverage by 2.12%.
The diff coverage is 83.33%.

@@             Coverage Diff             @@
##           develop       #9      +/-   ##
===========================================
+ Coverage    79.47%   81.59%   +2.12%     
===========================================
  Files           10       13       +3     
  Lines          151      201      +50     
  Branches         0        2       +2     
===========================================
+ Hits           120      164      +44     
- Misses          31       37       +6

Impacted Files	Coverage Δ
checklist/src/main/scala/checklist/Indexable.scala	100% <100%> (+33.33%)	⬆️
...list/src/main/scala/checklist/SizeableSyntax.scala	100% <100%> (ø)
checklist/src/main/scala/checklist/Sizeable.scala	100% <100%> (ø)
...ist/src/main/scala/checklist/IndexableSyntax.scala	66.66% <66.66%> (ø)
checklist/src/main/scala/checklist/Rule.scala	80.91% <80.85%> (-0.34%)	⬇️

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[Jacoby6000]

@davegurnell Hey dave, just checking to see if you saw this.

[davegurnell]

I have now! It all looks good except for one thing, which I think is a bigger design problem with the library in general that needs to be sorted in the medium term. I'll merge this now anyway and raise an issue to discuss the design problem.