Adds Semigroup for Xor

[mikejcurry]

As per #716, adding a Semigroup for the Xor data type which currently
has a Monoid instance but no Semigroup instance.

[ceedubs]

Minor: monoid/semigroup are already included in the test names, so I think the convention has been to not include Monoid/Semigroup in the string argument.

[mikejcurry]

I see - I can certainly change this. For some reason I'd thought I'd seen a mix of these versions, but I see now it was likely my eyes deceiving me. :-)

[ceedubs]

👍 if/when the build passes.

@mikejcurry you are on a roll again! :)

[codecov-io]

Current coverage is 85.49%

Merging #717 into master will increase coverage by +0.02% as of 1660921

@@            master    #717   diff @@
======================================
  Files          162     162       
  Stmts         2237    2240     +3
  Branches        74      74       
  Methods          0       0       
======================================
+ Hit           1912    1915     +3
  Partial          0       0       
  Missed         325     325       

Review entire Coverage Diff as of 1660921

Powered by Codecov. Updated on successful CI builds.

[mikejcurry]

Pushed a change to keep things in line with naming conventions for tests. Maybe I should add this convention to the contributors guide?

[ceedubs]

@mikejcurry that would be great! Thank you. I think the sole exception is the Cats serializability test, since the same one is used for all types. Actually, if you are already going to make a change, could you add that all type class instances should also be checked with the serializability tests unless they are being checked by Algebra's laws (GroupLaws, OrderLaws, etc), which already test serializability? This helps keep us Spark-friendly (and other tools that require java Serializability).

[mikejcurry]

Sure, have to head away for a while now, so it may not be today, but I'll get to it some time soon.

[adelbertc]

👍