Added support for LetDef with mutually recursive functions.

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

src/main/scala/leon/purescala/ExprOps.scala

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

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

src/main/scala/leon/purescala/FunctionClosure.scala

src/main/scala/leon/purescala/ScopeSimplifier.scala

src/main/scala/leon/purescala/TypeOps.scala

src/main/scala/leon/synthesis/Solution.scala

src/main/scala/leon/termination/SelfCallsProcessor.scala

src/main/scala/leon/transformations/StackSpacePhase.scala

src/main/scala/leon/utils/UnitElimination.scala

src/main/scala/leon/xlang/EpsilonElimination.scala

src/main/scala/leon/xlang/ImperativeCodeElimination.scala