Move Parallel instances for EitherT and OptionT into implicit scope

[travisbrown]

Fixes #3032. Now the following works:

scala> import cats.{Id, Parallel}, cats.instances.list._, cats.instances.string._
import cats.{Id, Parallel}
import cats.instances.list._
import cats.instances.string._

scala> type EI[A] = cats.data.EitherT[Id, String, A]
defined type alias EI

scala> implicitly[Parallel[EI]]
res0: cats.Parallel[EI] = cats.data.EitherTInstances$$anon$7@3f7c7017

scala> type EL[A] = cats.data.EitherT[List, String, A]
defined type alias EL

scala> implicitly[Parallel[EL]]
res1: cats.Parallel[EL] = cats.data.EitherTInstances1$$anon$15@1124d0e8

scala> type OI[A] = cats.data.OptionT[Id, A]
defined type alias OI

scala> implicitly[Parallel[OI]]
res2: cats.Parallel[OI] = cats.data.OptionTInstances$$anon$8@3e5db9dc

This should have no changes for existing code that doesn't refer to the two cats.instances.parallel instances by name (I've made them package-private, not just deprecated them). It doesn't break binary compatibility.

[codecov-io]

Codecov Report

Merging #3033 into master will decrease coverage by 0.13%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3033      +/-   ##
==========================================
- Coverage   93.56%   93.43%   -0.14%     
==========================================
  Files         368      368              
  Lines        6949     6975      +26     
  Branches      195      187       -8     
==========================================
+ Hits         6502     6517      +15     
- Misses        447      458      +11

Impacted Files	Coverage Δ
core/src/main/scala/cats/instances/parallel.scala	0% <ø> (-100%)	⬇️
core/src/main/scala/cats/data/EitherT.scala	98.18% <100%> (+0.25%)	⬆️
core/src/main/scala/cats/data/OptionT.scala	95.65% <100%> (+0.23%)	⬆️
testkit/src/main/scala/cats/tests/Helpers.scala	96% <0%> (-4%)	⬇️
...cala/cats/kernel/instances/FunctionInstances.scala	96.96% <0%> (-3.04%)	⬇️
core/src/main/scala/cats/Parallel.scala	84.37% <0%> (ø)	⬆️

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