Add XorT#fromEither

[markus1189]

Adds the ability to call XorT.fromEither to lift from Either[A,B] . This is sugar for the more verbose x => XorT(Xor.fromEither(x)) and complements the ability to go from XorT to Either via toEither

[codecov-io]

Current coverage is 88.38%

Merging #1023 into master will increase coverage by +<.01%

1. 3 files (not in diff) in .../main/scala/cats/std were modified. more
2. 2 files (not in diff) in ...n/scala/cats/functor were modified. more

@@             master      #1023   diff @@
==========================================
  Files           215        215          
  Lines          2702       2703     +1   
  Methods        2633       2638     +5   
  Messages          0          0          
  Branches         64         60     -4   
==========================================
+ Hits           2388       2389     +1   
  Misses          314        314          
  Partials          0          0          

Powered by Codecov. Last updated by 4285a07...4017e72

[adelbertc]

👍 on green

[ceedubs]

👍 thanks!