Updated the way abstract examples are generated w.r.t. the new datagen

Merged modifications in QuestionBuilder
Better imports in StringRender.scala

# Conflicts:
#	src/main/scala/leon/synthesis/ExamplesFinder.scala
#	src/main/scala/leon/synthesis/disambiguation/ExamplesAdder.scala
#	src/main/scala/leon/synthesis/disambiguation/QuestionBuilder.scala
#	src/main/scala/leon/synthesis/rules/StringRender.scala