Add Applicative.unit

[alexandru]

Adding a short-hand to Applicative for F.pure(()).

@typeclass trait Applicative[F[_]] extends Apply[F] { self =>
  /**
   * Returns an `F[Unit]` value, equivalent with `pure(())`.
   *
   * A useful shorthand, also allowing implementations to optimize the
   * returned reference (e.g. it can be a `val`).
   */
  def unit: F[Unit] = pure(())

Reasons:

1. I find myself doing F.pure(()).map(_ => a) a lot (due to pure not taking a by-name param and having no way to do that otherwise, but that's another discussion)
2. F[Unit] is often used to indicate callbacks that are going to trigger side-effects, like for resource clean-ups - for example in Task.doOnFinish and we often need to create empty F[Unit] instances that don't do anything; F.pure(()) is more noisy versus F.unit
3. Applicative implementations can optimize this to always return the same reference, instead of rebuilding pure(()) over and over again

Prior art:

scala.concurrent.Future.unit

[mpilquist]

👍

[codecov-io]

Codecov Report

Merging #1586 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1586      +/-   ##
==========================================
+ Coverage   92.42%   92.42%   +<.01%     
==========================================
  Files         249      249              
  Lines        3971     3974       +3     
  Branches      146      131      -15     
==========================================
+ Hits         3670     3673       +3     
  Misses        301      301

Impacted Files	Coverage Δ
.../scala/cats/laws/discipline/ApplicativeTests.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/Applicative.scala	75% <100%> (+2.27%)	⬆️
...aws/src/main/scala/cats/laws/ApplicativeLaws.scala	100% <100%> (ø)	⬆️

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