Fix the problem with unstable names synthesized for existential
types (declared with underscore syntax) by renaming type variables
to a scheme that is guaranteed to be stable no matter where given
the existential type appears.

We can do that because type variables declared in existential types
are scoped to those types so the renaming operation is rather local.

There's a specs2 unit test covering instability of existential types.
The test is included in compiler-interface project and the build
definition has been modified to enable building and executing tests
in compiler-interface project. Some dependencies has been modified:

  * compiler-interface project depends on api project for testing
    (test makes us of SameAPI)
  * dependency on junit has been introduced because it's needed
    for `@RunWith` annotation which declares that specs2 unit
    test should be ran with JUnitRunner

SameAPI has been modified to expose a method that allows us to
compare two definitions.

Also, a integration test has been added which tests scenario
mentioned in sbt#823.

This commit fixes sbt#823.