Move Parallel instances into instance packages for the type constructors they characterize

[travisbrown]

It seems pretty reasonable to expect the following to work:

import cats.instances.either._, cats.instances.string._, cats.syntax.parallel._

val e: Either[String, Int] = Left("e")

(e, e).parTupled

But it doesn't, because the Parallel instance for Either is in cats.instances.parallel, not cats.instances.either

After #3033, the only instances in the parallel instance package are for Either, Stream, and the NonEmptyParallel instances for List and Vector. I think all of these should be moved the instance packages for those types—cats.instances.either, cats.instances.stream.

This change would break source compatibility, but could be done in a binary-compatible way in 2.1 (by making all of the contents of cats.instance.parallel non-implicit and deprecated).

[barambani]

If the work on this hasn't started yet I'd like to give a hand.

[travisbrown]

@barambani I've not done anything on it, so that sounds good to me! I think it's a pretty reasonable change that could happen in 2.1, but we'll want to make sure there's consensus about that.

[barambani]

Makes sense, I will wait for another while then, to give time to the discussion. Thanks 👍