Reasonable scala

Recognizing scalac is slow no matter how you engineer around it and the fact that zinc integration may complicate the bazel model here is another option to keep an eye on https://github.com/twitter/reasonable-scala

Start by reading the issue text and the linked reasonable-scala project. Determine whether its approach fits the rules_scala Bazel model and define the scope of any proposed integration; the issue is complete only when that direction and concrete next steps are settled.

Written by the indexing model from the issue text.