Implement lambda expressions

This is WIP. This are some of the questions I have:

* What should I return in remapLambda: should I create a copy or just return the node?
* Should I try to create a unique name for the lambda as I currently try (AST_Lambda::name) or just return always "lambda"?
* SourceInfo::getBody() creates an AST_Return which is a hack which will probably lead to a memory leak but on the positive side it makes lambda expressions possible without much code changes. what todo with it?

Please carefully review this patch. :-)