Resolves typelevel#888

I don't think there's really a _right_ answer here. Both methods of
combining `Xor`s are straightforward and law-abiding. However, I think
that since in pretty much every other context (including the
`SemigroupK` instances), `Xor` does not combine failures and `Validated`
does, it is less surprising behavior to not combine left values in
`combine` methods for `Xor` and `XorT`.

Some notes about related implementations:
- Scalaz's respective methods (and semigroup instances) _do_ combine
  errors, so this is a deviation from that.
- The `Semigroup` for `Either` in Haskell has doesn't combine values at
  all, but returns the first `Right` (if any), making it equivalent to
  the behavior of `orElse` and the `SemigroupK` instance for `Xor`.
  Since we have already decided to not go with `orElse`-like behavior
  for our `Option` semigroup, I'm inclined to not take this approach for
  `Xor`.

See also
typelevel#996 (comment)

Per @non's suggestion.