add Scala 3 to crossbuild

[SethTisue]

includes commits from #393 — I'll rebase this one once that one's merged

references #390

[SethTisue]

status: this is still in early stages, I am still just chewing through bread and have yet to take a bite of meat

[sullis]

ScalaTest 3.2.5

[SethTisue]

Travis-CI config seems messed up, there is Scala 3 stuff in the Scala 2 jobs...

[SethTisue]

just a note that I haven't forgotten about this. I've been working on play-json instead, but this is next in line.

[SethTisue]

Travis-CI config seems messed up, there is Scala 3 stuff in the Scala 2 jobs

Fixed now. But there are still two non-Scala 3 problems.

The plugin test job fails with Conflicting cross-version suffixes

And also, MiMa is failing:

[error] twirl-parser: Failed binary compatibility check against com.typesafe.play:twirl-parser_2.12:1.5.0! Found 1 potential problems
[error]  * method several(scala.Function0,scala.collection.mutable.Buffer,scala.reflect.Manifest)scala.collection.mutable.Buffer in class play.twirl.parser.TwirlParser's type is different in current version, where it is (scala.Function0,scala.collection.mutable.Buffer,scala.reflect.ClassTag)scala.collection.mutable.Buffer instead of (scala.Function0,scala.collection.mutable.Buffer,scala.reflect.Manifest)scala.collection.mutable.Buffer
[error]    filter with: ProblemFilters.exclude[IncompatibleMethTypeProblem]("play.twirl.parser.TwirlParser.several")

[SethTisue]

me up in .travis.yml trying to understand how matrix: and stage: interact:

[SethTisue]

Man, I hate builds that have an sbt plugin subproject mixed in with crossbuilt subprojects. It makes sense for such builds to exist, but 🙀

[SethTisue]

I think I got the CI matrix sorted out, including the plugin tests. I don't like omitting plugin from the root project aggregate, but I don't know how to improve that.

So what remains here is:

• MiMa is failing because Manifest vs ClassTag. I guess we ought to just bite the bullet here and get off Manifest, even if it means accepting the binary incompatibility and bumping the major version?
• the actual Scala 3 work

[SethTisue]

Still chipping away at this. A lot of PRs (mostly version bumps) were merged since I initially submitted this, so I rebased. And then I made separate scala-2 and scala-3 source directories with just the version-dependent bits in them, but right now the Scala 3 ones are just ??? stubs.

[SethTisue]

abandoning; someone else is free to pick it up

[SethTisue]

some of the context for this is at https://discuss.lightbend.com/t/github-project-abandonned/8781/12, by the way