splitString/Choice are constrained to categories while they do not require identity

[erykciepiela]

splitStrong
  :: forall p a b c d
   . Category p -- actually only Semigroupoid required 
  => Strong p
  => p a b
  -> p c d
  -> p (Tuple a c) (Tuple b d)
splitStrong l r = first l >>> second r

splitChoice
  :: forall p a b c d
   . Category p -- actually only Semigroupoid required 
  => Choice p
  => p a b
  -> p c d
  -> p (Either a c) (Either b d)
splitChoice l r = left l >>> right r

[erykciepiela]

Closing as it's already merged.