Separate free package into its own module

It's quite possible that people are going to throw tomatoes at me for
proposing this change. There has been much discussion about
modularization in the past, and in fact free used to have its own
module, which was removed in typelevel#760/typelevel#765.

One significant change that has happened since then is that `State` has
started to use `Eval` instead of `Trampoline` for trampolining. At this
point, nothing outside of the `free` package in `cats-core` depends on
`free` (which is why this PR was so easy to create). To me that makes it
a fairly convenient border for modularization.

There was a [brief
discussion](https://gitter.im/typelevel/cats?at=5716322b548df1be102e3f3c) about this on Gitter in which @sellout supported this change on the basis that they prefer to use fixpoint types (such as `Mu`) for free structures. Putting `free` into a separate module may be better for people who want to use different variations of these structures.

Of course in addition to these more recent updates, there are the usual
arguments for modularization and decreased jar size (which I believe is
especially important for `catsJS` users).

build.sbt

@@ -97,7 +97,7 @@ def noDocProjects(sv: String): Seq[ProjectReference] = CrossVersion.partialVersi
 lazy val docSettings = Seq(
   autoAPIMappings := true,
   unidocProjectFilter in (ScalaUnidoc, unidoc) :=
-    inProjects(coreJVM) -- inProjects(noDocProjects(scalaVersion.value): _*),
+    inProjects(coreJVM, freeJVM) -- inProjects(noDocProjects(scalaVersion.value): _*),
   site.addMappingsToSiteDir(mappings in (ScalaUnidoc, packageDoc), "api"),
   site.addMappingsToSiteDir(tut, "_tut"),
   ghpagesNoJekyll := false,
@@ -123,7 +123,7 @@ lazy val docs = project
   .settings(tutSettings)
   .settings(tutScalacOptions ~= (_.filterNot(Set("-Ywarn-unused-import", "-Ywarn-dead-code"))))
   .settings(commonJvmSettings)
-  .dependsOn(coreJVM)
+  .dependsOn(coreJVM, freeJVM)
 
 lazy val cats = project.in(file("."))
   .settings(moduleName := "root")
@@ -136,15 +136,15 @@ lazy val catsJVM = project.in(file(".catsJVM"))
   .settings(moduleName := "cats")
   .settings(catsSettings)
   .settings(commonJvmSettings)
-  .aggregate(macrosJVM, coreJVM, lawsJVM, testsJVM, jvm, docs, bench)
-  .dependsOn(macrosJVM, coreJVM, lawsJVM, testsJVM % "test-internal -> test", jvm, bench % "compile-internal;test-internal -> test")
+  .aggregate(macrosJVM, coreJVM, lawsJVM, freeJVM, testsJVM, jvm, docs, bench)
+  .dependsOn(macrosJVM, coreJVM, lawsJVM, freeJVM, testsJVM % "test-internal -> test", jvm, bench % "compile-internal;test-internal -> test")
 
 lazy val catsJS = project.in(file(".catsJS"))
   .settings(moduleName := "cats")
   .settings(catsSettings)
   .settings(commonJsSettings)
-  .aggregate(macrosJS, coreJS, lawsJS, testsJS, js)
-  .dependsOn(macrosJS, coreJS, lawsJS, testsJS % "test-internal -> test", js)
+  .aggregate(macrosJS, coreJS, lawsJS, freeJS, testsJS, js)
+  .dependsOn(macrosJS, coreJS, lawsJS, freeJS, testsJS % "test-internal -> test", js)
   .enablePlugins(ScalaJSPlugin)
 
 
@@ -187,6 +187,16 @@ lazy val laws = crossProject.crossType(CrossType.Pure)
 lazy val lawsJVM = laws.jvm
 lazy val lawsJS = laws.js
 
+lazy val free = crossProject.crossType(CrossType.Pure)
+  .dependsOn(macros, core, tests % "test-internal -> test")
+  .settings(moduleName := "cats-free")
+  .settings(catsSettings:_*)
+  .jsSettings(commonJsSettings:_*)
+  .jvmSettings(commonJvmSettings:_*)
+
+lazy val freeJVM = free.jvm
+lazy val freeJS = free.js
+
 lazy val tests = crossProject.crossType(CrossType.Pure)
   .dependsOn(macros, core, laws)
   .settings(moduleName := "cats-tests")
@@ -210,7 +220,7 @@ lazy val jvm = project
   .settings(commonJvmSettings:_*)
 
 // bench is currently JVM-only
-lazy val bench = project.dependsOn(macrosJVM, coreJVM, lawsJVM)
+lazy val bench = project.dependsOn(macrosJVM, coreJVM, freeJVM, lawsJVM)
   .settings(moduleName := "cats-bench")
   .settings(catsSettings)
   .settings(noPublishSettings)

tests/src/test/scala/cats/tests/CoyonedaTests.scala → free/src/test/scala/cats/free/CoyonedaTests.scala

@@ -1,8 +1,8 @@
 package cats
-package tests
+package free
 
+import cats.tests.CatsSuite
 import cats.arrow.NaturalTransformation
-import cats.free.Coyoneda
 import cats.laws.discipline.{FunctorTests, SerializableTests}
 
 import org.scalacheck.Arbitrary

tests/src/test/scala/cats/tests/FreeApplicativeTests.scala → free/src/test/scala/cats/free/FreeApplicativeTests.scala

@@ -1,8 +1,8 @@
 package cats
-package tests
+package free
 
+import cats.tests.CatsSuite
 import cats.arrow.NaturalTransformation
-import cats.free.FreeApplicative
 import cats.laws.discipline.{CartesianTests, ApplicativeTests, SerializableTests}
 import cats.laws.discipline.eq.{tuple3Eq, tuple2Eq}
 import cats.data.State

tests/src/test/scala/cats/tests/FreeTests.scala → free/src/test/scala/cats/free/FreeTests.scala

@@ -1,8 +1,8 @@
 package cats
-package tests
+package free
 
+import cats.tests.CatsSuite
 import cats.arrow.NaturalTransformation
-import cats.free.{Free, Trampoline}
 import cats.laws.discipline.{CartesianTests, MonadTests, SerializableTests}
 import cats.laws.discipline.eq._
 import cats.laws.discipline.arbitrary.function0Arbitrary

tests/src/test/scala/cats/tests/InjectTests.scala → free/src/test/scala/cats/free/InjectTests.scala

@@ -1,8 +1,8 @@
 package cats
-package tests
+package free
 
+import cats.tests.CatsSuite
 import cats.data.{Xor, Coproduct}
-import cats.free.{Free, Inject,:<:}
 import org.scalacheck._
 
 class InjectTests extends CatsSuite {

tests/src/test/scala/cats/tests/YonedaTests.scala → free/src/test/scala/cats/free/YonedaTests.scala

@@ -1,7 +1,7 @@
 package cats
-package tests
+package free
 
-import cats.free.Yoneda
+import cats.tests.CatsSuite
 import cats.laws.discipline.{FunctorTests, SerializableTests}
 
 import org.scalacheck.Arbitrary