Add compareTo overload for Order

[Mishkun]

This pull-request provides compareTo operators for all Order instances as described in #729.
There are two more operators mentioned in #729:

• Equality operator (==, !=) for Eq instances - it is impossible to implement that as extension function, because member functions always shadow extensions and equals is defined for every object in Kotlin (because of Java legacy)
• Plus operator (+) for Semigroup instances - _already added in Semigroup + syntax #768 _

It seems that all three items of issue would resolved after this PR is merged, so it will close #729

[nomisRev]

Should we replace compare by compareTo since they're the same function?

This would also make a nice example of the usefulness of typeclasses. Since many people are familiar with the compareTo operator magic, we can explain how you could do it his easily with typeclasses yourself.

[codecov]

Codecov Report

Merging #1035 into master will decrease coverage by 0.02%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##             master   #1035      +/-   ##
===========================================
- Coverage     47.72%   47.7%   -0.03%     
  Complexity      725     725              
===========================================
  Files           333     333              
  Lines          8479    8483       +4     
  Branches        771     772       +1     
===========================================
  Hits           4047    4047              
- Misses         4143    4147       +4     
  Partials        289     289

Impacted Files	Coverage Δ	Complexity Δ
...-test/src/main/kotlin/arrow/test/laws/OrderLaws.kt	0% <0%> (ø)	0 <0> (ø)	⬇️
...classes/src/main/kotlin/arrow/typeclasses/Order.kt	0% <0%> (ø)	0 <0> (ø)	⬇️

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[pakoito]

For consistency with the other typeclasses, I'd keep them separated for now. We can still use it as an example, and deprecate later.

@Mishkun Thanks for the PR, well done adding the laws and docs too :)