Rename Inject to InjectK

[andyscott]

Rename Inject to InjectK to be consistent with names/renames for EitherK, FunctionK, Tuple2K, and friends. See #1589.

This is a breaking API and breaking binary change.

Additionally, this opens the door to "re-introduce" Inject for Either:

sealed abstract class Inject[A, B] {
  def inj: A => B
  def prj: B => Option[A]
  def apply(a: A): B = inj(a)
  def unapply(b: B): Option[A] = prj(b)
}

private[cats] sealed abstract class InjectInstances {
  implicit def catsReflexiveInjectInstance[A]: Inject[A, A] =
    new Inject[A, A] {
      val inj = identity
      val prj = Some(_)
    }

  implicit def catsLeftInjectInstance[A, B]: Inject[A, Either[A, B]] =
    new Inject[A, Either[A, B]] {
      val inj = Left(_)
      val prj = _.left.toOption
    }

  implicit def catsRightInjectInstance[A, B, C](implicit I: Inject[A, B]): Inject[A, Either[C, B]] =
    new Inject[A, Either[C, B]] {
      val inj = a => Right(I.inj(a))
      val prj = _.right.toOption.flatMap(I.prj)
    }

}

object Inject extends InjectInstances {
  def apply[A, B](implicit I: Inject[A, B]): Inject[A, B] = I
}

Inject for Either would be useful when combining error algebras across domains with Free.

[codecov-io]

Codecov Report

Merging #1596 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1596   +/-   ##
=======================================
  Coverage   93.11%   93.11%           
=======================================
  Files         250      250           
  Lines        3992     3992           
  Branches      138      136    -2     
=======================================
  Hits         3717     3717           
  Misses        275      275

Impacted Files	Coverage Δ
core/src/main/scala/cats/package.scala	100% <ø> (ø)	⬆️
free/src/main/scala/cats/free/Free.scala	88.23% <100%> (ø)	⬆️
core/src/main/scala/cats/InjectK.scala	100% <100%> (ø)

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[kailuowang]

Makes sense to me. 👍 Can we also create a ticket for the new Inject? It would be a bit awkward to have InjectK but not Inject.

[andyscott]

Sure, and I'll put up a PR up for Inject if this one gets merged.

[ceedubs]

Thanks @andyscott this looks good to me. I only have one minor suggestion: there are a couple places that now say "InjectK type class as described in "Data types a la carte". This might be slightly confusing to readers, since that paper doesn't mention an InjectK. Maybe we could just change this text to "The injection type class as described..." or something?

[andyscott]

@ceedubs Good call, I'll address that now

[ceedubs]

Thanks @andyscott! 👍 LGTM.