Added Align instances for Id and Kleisli

[atnoya]

This PR adds Align instances for Kleisli and Id.

I realized these two were not available while trying some code like (simplified):

type Env = Map[String, String]
type FromEnvPrefixed[A] = Reader[Env, A]

def prefixed(prefix: String): Reader[Env, Map[String, String]] = {
   Reader { (env: Env) => 
      env.filter(_._1.startsWith(prefix)).foldLeft(Map.empty[String, String]) { case (acc, (k, v)) =>
            val tKey = k.replace(prefix, "")
            acc + (tKey, v)
      }
   }
}

val inConfig = prefixed("label").align(prefixed("label2")).value.run(Map("labelprop1" -> "prop1", "label2prop1" -> "prop1", "labelprop2" -> "prop2")

inConfig.toList.traverse {
   case (k, Ior.Both(one, two)) => Right(k -> (one, two)) //Yeah!!
   case (k, _) => Left(s"Missing property for key '$k'") //Missing for one of the prefixes
}

Looking at docs and laws I see no reason why they shouldn't have it. Please let me know otherwise.

[atnoya]

For some reason this is required for scala 2.12.x, as it fails to find the implicit Align for Kleisli on its own. Is there another way to do this?

[armanbilge]

I guess it must be an inference issue. /shrug

[error] /workspace/cats/tests/shared/src/test/scala/cats/tests/KleisliSuite.scala:93:51: could not find implicit value for evidence parameter of type cats.Align[[γ$23$]cats.data.Kleisli[[A]A,cats.laws.discipline.MiniInt,γ$23$]]
[error]     checkAll("Kleisli[Id, MiniInt, *]", AlignTests[Kleisli[Id, MiniInt, *]].align[Int, Int, Int, Int])
[error]                                                   ^

[armanbilge]

LGTM. So sorry for the slow review cycle.

[armanbilge]

I think this is a stack-safety trap, but it's not obvious to me if there's a better way.

[atnoya]

LGTM. So sorry for the slow review cycle.

No worries, definitely not urgent, appreciate your dedication to OSS!

Should I go ahead and merge or wait for more reviewers?

[satorg]

Looks good to me too, thanks!