Adds Monoid instance for Validated

[mikejcurry]

Adds a Monoid instance for Validated

[codecov-io]

Current coverage is 85.45%

Merging #715 into master will increase coverage by +0.03% as of 1faff73

@@            master    #715   diff @@
======================================
  Files          162     162       
  Stmts         2230    2234     +4
  Branches        74      74       
  Methods          0       0       
======================================
+ Hit           1905    1909     +4
  Partial          0       0       
  Missed         325     325       

Review entire Coverage Diff as of 1faff73

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[adelbertc]

👍

[ceedubs]

👍 thanks!

It would also be nice to have a Semigroup instance if B has a Semigroup but not a Monoid.