Add javaapi collection forwarders for 2.12 and 2.11

[DieBauer]

This PR adds 2.13 collection compatible forwarders for scala 2.11 and scala 2.12, see #346.

2.11 has converters in traits WrapAsJava and WrapAsScala, while 2.12 has them in AsJavaConverters and AsScalaConverters.
The method names are the same, but unfortunately both CollectionConverters objects have to be separately defined for both sources, because of this import.

Tests are added in a Java file, to verify that the methods can be called as java api.

[SethTisue]

review by @scala/collections ?

[SethTisue]

Well, this certainly looks plausible to me. Anyone else want to take a look? Otherwise I guess we'll optimistically merge.

[SethTisue]

@DieBauer headerCreate should fix the headerCheck part of the sbt failure (it's from https://github.com/sbt/sbt-header), plus it looks like you need to run scalafmt on the new sources.

[DieBauer]

Thanks for the hints! headers added, and sources formatted according to scalafmt, and squashed in a single commit

[DieBauer]

The 2.11 and 2.12 build fails the versionPolicyMimaCheck. @SethTisue I'm not sure what to do here?

[error] scala-collection-compat: Failed binary compatibility check against org.scala-lang.modules:scala-collection-compat_2.11:2.8.1! Found 2 potential problems
[error]  * class scala.jdk.javaapi.CollectionConverters does not have a correspondent in other version
[error]    filter with: ProblemFilters.exclude[MissingClassProblem]("scala.jdk.javaapi.CollectionConverters")
[error]  * object scala.jdk.javaapi.CollectionConverters does not have a correspondent in other version
[error]    filter with: ProblemFilters.exclude[MissingClassProblem]("scala.jdk.javaapi.CollectionConverters$")
[error] Module scala-collection-compat:2.8.1+3-64b881ac-SNAPSHOT is not source compatible with version 2.8.1. You have to relax your compatibility intention by changing the value of versionPolicyIntention.
...
[error] java.lang.RuntimeException: Failed binary compatibility check against org.scala-lang.modules:scala-collection-compat_2.11:2.8.1! Found 2 potential problems
[error] (compat211 / versionPolicyMimaCheck) java.lang.RuntimeException: Failed binary compatibility check against org.scala-lang.modules:scala-collection-compat_2.11:2.8.1! Found 2 potential problems

[julienrf]

You need to change the versionPolicyIntention to Compatibility.BinaryCompatible.

[DieBauer]

I see the policy was changed with this PR: #553
Can that be reverted in this PR, or as a new one to master?

[SethTisue]

in this PR is fine, but in its own commit is best

[SethTisue]

Thank you!