add parallel collections module to 2.13 build and fix key projects to use it

[SethTisue]

once scala/scala#5603 is merged

as per discussion on that PR

[SethTisue]

I hit merge on 5603 just now, so I can start on this tomorrow if tonight's bootstrap job is green

[SethTisue]

Welcome to Scala 2.13.0-pre-e2a2cba (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_112).
Type in expressions for evaluation. Or try :help.

scala> List(1).par
<console>:12: error: value par is not a member of List[Int]
       List(1).par
               ^

ahhhhhhh

[SethTisue]

[scala-parallel-collections] [error] Uncaught exception when running scala.runtime.ScalaRunTimeTest: java.lang.NoClassDefFoundError: org/hamcrest/SelfDescribing
[scala-parallel-collections] sbt.ForkMain$ForkError: java.lang.NoClassDefFoundError: org/hamcrest/SelfDescribing

🤔

[SethTisue]

pushed a fix for the hamcrest thing (though I don't understand it)

[SethTisue]

fixed by #501

[SethTisue]

reopening because there's still the "and fix key projects to use it" part left to do

[SethTisue]

[info] >>> The dbuild result is------------: FAILED (failed: scala-js, scala-java8-compat, fs2, scala-stm, jackson-module-scala, specs2)

37 projects remain green. scala-js and specs2 are the heavies here.