Added string escape

library/lang/StrOps.scala

src/main/java/leon/codegen/runtime/StrOps.java

src/main/scala/leon/codegen/CodeGeneration.scala

src/main/scala/leon/datagen/VanuatooDataGen.scala

src/main/scala/leon/evaluators/RecursiveEvaluator.scala

src/main/scala/leon/evaluators/StringTracingEvaluator.scala

src/main/scala/leon/frontends/scalac/ASTExtractors.scala

src/main/scala/leon/frontends/scalac/CodeExtraction.scala

src/main/scala/leon/grammars/ValueGrammar.scala

src/main/scala/leon/purescala/Expressions.scala

src/main/scala/leon/purescala/Extractors.scala

src/main/scala/leon/purescala/PrettyPrinter.scala

src/main/scala/leon/purescala/ScalaPrinter.scala

src/main/scala/leon/synthesis/rules/StringRender.scala