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:

THE FOLLOWING DISCLAIMER APPLIES TO ALL SOFTWARE CODE AND OTHER MATERIALS CONTRIBUTED IN CONNECTION WITH THIS SOFTWARE:
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I'm trying to understand how much code duplication there was... So, if I understand correctly, companions provide implementations for 
```
  def apply(context, underlying)
  def mirror(value, context)
```
so it seems like they could be abstract methods in `BoxedJdiProxy`. F-bounded types are hard to understand, and in my opinion unncessary.