add Scala 2.13.0-M5 to cross-build

[SethTisue]

No description provided.

[alexarchambault]

ping @lihaoyi

[lihaoyi]

@alexarchambault gave you commit access, since you seem to be doing a lot of 213 related stuff feel free to go wild

[alexarchambault]

@SethTisue Couldn't we manage not to rely on sbt-buildinfo, or at least not have it add public stuff in the main module? The scala version is already available via scala.util.Properties.versionNumberString (may require forking in tests to work right), and the cross stuff could be deduced via platform-specific sources under geny/{js, jvm}/src/.

[lihaoyi]

Honestly I'd just be happy with it saying x == "WrappedArray(...)" || x == "WrappedVarArgs(...)". The chances of that looseness causing a regression to slip through are just about zero

[ajrnz]

This should be implemented by #7 - shout if not