Expression evaluator for Scala IDE

Expression evaluator for Scala IDE

Implementation of expression evaluator and conditional breakpoints based on JDI.

Conditional breakpoints could be set in standard Eclipse debug window, for expression evaluator there is separate view. Both work with "new Scala" debug launcher.

Both allows user to evaluate any Scala expression in context of some breakpoint during debug. Expression is transformed in multiple phases and evaluated using calls to remote debugged machine and result is fetched back to user.

Because of complexity of Scala language, implementation details of JVM platform and necessity for manual evaluation not all Scala features are supported yet. Full list of features is available in `EXPRESSIONS.md` file.

Because of huge size of this PR we will contact you about making an introduction call to what had been done and how we approached it.

Lastly, the below disclaimer is required by the lawyers:

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To expand on my previous comment. It's possible to have more than one thread suspended at a given time. The user may select any one of them, and that should probably be reflected in the evaluator (in the same spirit, but way more common, users can select a different stack frame in a thread).

These selections are part of a *debug context*, and you can get notified using a `DebugContextManager.getDefault().addDebugContextListener`. However, that's part of the UI, so you need to be a bit careful when running tests. Have a look at how we do it in `ScalaDebugger` (where we also maintain `currentThread`).