Adds Semigroup for Const

[mikejcurry]

Adding a Semigroup for Const to allow Semigroup behaviour for data
types with no valid Monoid instance.

[codecov-io]

Current coverage is 85.48%

Merging #718 into master will increase coverage by +0.01% as of ebbc614

@@            master    #718   diff @@
======================================
  Files          162     162       
  Stmts         2237    2239     +2
  Branches        74      74       
  Methods          0       0       
======================================
+ Hit           1912    1914     +2
  Partial          0       0       
  Missed         325     325       

Review entire Coverage Diff as of ebbc614

Powered by Codecov. Updated on successful CI builds.

[non]

Looks good to me. 👍

[ceedubs]

👍