Rename SemigroupK.combine to combineK

[ceedubs]

Fixes #763.

[codecov-io]

Current coverage is 88.47%

Merging #781 into master will not affect coverage as of 3e37007

@@            master    #781   diff @@
======================================
  Files          166     166       
  Stmts         2291    2291       
  Branches        75      75       
  Methods          0       0       
======================================
  Hit           2027    2027       
  Partial          0       0       
  Missed         264     264       

Review entire Coverage Diff as of 3e37007

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[ceedubs]

@travisbrown since you mentioned that you'd like to see this in the next Cats release, could you be so kind as to review it? :)

[mpilquist]

👍

[travisbrown]

👍