reinstate Scala Native support?

[SethTisue]

previously: #118, #132, #135

[SethTisue]

should we try to recruit a volunteer, e.g. by posting on https://contributors.scala-lang.org, the Scala Native gitter channel, etc?

[SethTisue]

this won't become a "good first issue" (for someone who knows the basics of Scala Native builds in sbt, I mean) until we get on sbt 1, I think.