Adding Evaluation and code generation for strings.

library/lang/package.scala

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

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

src/main/scala/leon/codegen/CompilationUnit.scala

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

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

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

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

testcases/stringrender/IntWrapperRender.scala