Xor's SemigroupK instance follows that of XorT.

We remove the MonoidK instance as it depended on an not-well defined
SemigroupK instance. The new SemigroupK instance places no constraints
on `L`, and has a "first Right or Last Left" behaviour for combine. This
has a closer semantic meaning to Xor(T)'s "success or failure"
interpretation. It didn't make sense previously that the empty value for
XortT's MonoidK instance represented failure
(`XortT.left(F.pure(L.empty))(F)`). Why should every empty value signal
failure?

Update Xor's SemigroupK instance to be aligned with XorT's, with
first-right, last-left semnatics.