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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Usually you use a symbol for that. So, you'd need to compare `tree.symbol` with `symbolForPlaceholder`. You could try to obtain one via reflection, something like `typeOf[JdiContext]` and then look through its members. For some examples, check the Scala compiler standard definitions, where symbols for "basic" things, such as `scala.List` or `Nil` are retrieved.