Allow operation without a filesystem

[stephenjudkins]

Currently, input and output files are specified in the API in terms of java.io.File. The underlying Scala compiler, however, does not require interaction with the filesystem. If inputs and outputs could be handled through in-memory data structures it could open the door for some optimizations in some cases (in this case, in the https://github.com/higherkindness/rules_scala project).

I'm aware this is a large issue, but figured I'd open it to signal support.

[eed3si9n]

@stephenjudkins Thanks. I am sort of working on this here - https://github.com/sbt/zinc/pull/712

[stephenjudkins]

This is wonderful, I'm glad to know it's being worked on.

[eed3si9n]

@stephenjudkins I now have tests passing with in-memory source code. I don't know how difficult it would be to abstract the JAR files, but I feel like I should just allow JAR files to be.