Publish for Scala 3.0.0-RC1

[SethTisue]

after #357

Primary motivations: support Scala 3.0.0-RC1, Scala Native 0.4

• wait for scala-collection-compat to publish
  • Publish for Scala 3.0.0-RC1 scala-collection-compat#428
• merge PRs
  • Update scala to 3.0.0-RC1 #357
• choose version number
  • there aren't code changes, but there are dependency updates, so let's go with 1.2.0-M2
• tag the release and write release notes
  • https://github.com/scala/scala-parser-combinators/releases/tag/v1.2.0-M2
• wait for release build
  • https://travis-ci.com/github/scala/scala-parser-combinators/builds/217313353
• release the staging repos
  • https://oss.sonatype.org/#stagingRepositories
• artifacts on Maven Central
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-parser-combinators_3.0.0-RC1/1.2.0-M2/
• any README edits needed
  • f632d18