Provide compilation progress tracking API

[scabug]

We should provide an API that allows tools that embed the compiler (sbt, IDEs, Maven, etc.) to track progress of compilation. At minimal level we should expose the following information:

• current phase
• current compilation unit

I set Scala 2.11.0-M2 as fix version which expresses my ambition not a hard requirement.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-6985?orig=1
Reporter: @gkossakowski

[scabug]

@adriaanm said:
sbt/sbt#643

[scabug]

@adriaanm said:
Unassigning and rescheduling to M6 as previous deadline was missed.

[scabug]

@gkossakowski said:
Looking at this ticket now I'm not sure if there's anything that's needs to be done on Scala compiler side. Incremental compiler does provide progress tracking API (as indicated in sbt/#643 ticket) and it's being used in Eclipse. We probably clean up the way it's implemented but this would be part of bigger clean up of incremental compiler<->Scala compiler interaction. I think it makes sense to just close this ticket.