Use Tensor `Ring` instance instead of `fmap (*)` for CycRep

[crockeea]

Additive and Ring instances for CycRep currently define (*) = fmap (*). This could be (is?) much slower than letting individual Tensor instances define their own instances. Instead, we should add an entailment for Additive and Ring (any others?) so that CycRep's instances simply call the corresponding tensor instance.