Modularize partest

partest moves to https://github.com/scala/scala-partest

As partest is now resolved from maven, `test/partest` uses `ant test.suite.init`
to determine the classpath (serialized to build/pack/partest.properties)
that's necessary to run `scala.tools.partest.nest.ConsoleRunner`.

Thus, partest gets exactly the same classpath, whether run from
the command line through `test/partest` or via `ant test`.

The version of partest we're using is specified by
properties defined in versions.properties (formerly `starr.number`).

Currently, we're using:

```
scala.binary.version=2.11.0-M4
partest.version.number=1.0-RC3
```

NOTES:
  - The version of Scala being tested must be backwards binary compatible with
    the version of Scala that was used to compile partest.
  - Once 2.11 goes final, `scala.binary.version=2.11`, and `starr.version=2.11.0`.
  - Need scalacheck on classpath for test/partest scalacheck tests.
  - Removed atrophied ant tests (haven't been run/changed for at least two years
    I checked 81d6591 as a "random" sample).
  - Removed scalacheck. It's resolved as a partest dependency.
  - For now, use a locally built scalap
  - Kept the trace macro in the main repo (partest-extras)
  - New targets for faster pr validation: test-core-opt, test-stab-opt

TODO:
  - Use maven provided dependencies. I'll improve this in a coming version. Need to figure out how to do this in SBT.  This PR represents a significant amount of work and we're running out of time. This can be done independently when publishing partest.