Introduce compilation units

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

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

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

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

...in/scala/leon/purescala/CompleteAbstractDefinitions.scala

src/main/scala/leon/purescala/Definitions.scala

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

src/main/scala/leon/purescala/MethodLifting.scala

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

src/main/scala/leon/purescala/RestoreMethods.scala

src/main/scala/leon/synthesis/Synthesizer.scala

src/main/scala/leon/termination/Processor.scala

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

src/test/scala/leon/test/purescala/DataGen.scala

src/test/scala/leon/test/solvers/z3/FairZ3SolverTests.scala

.../scala/leon/test/solvers/z3/FairZ3SolverTestsNewAPI.scala

...ala/leon/test/solvers/z3/UninterpretedZ3SolverTests.scala