Support for calling @extern functions

build.sbt

src/main/scala/leon/LeonContext.scala

src/main/scala/leon/Main.scala

src/main/scala/leon/codegen/runtime/RuntimeResources.scala

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

src/main/scala/leon/evaluators/ScalacEvaluator.scala

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

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

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

src/main/scala/leon/purescala/DefOps.scala

testcases/extern/Editor.scala

testcases/extern/EditorSimple.scala

testcases/extern/Game.scala

testcases/extern/GameDemo.scala

testcases/extern/GameNoSet.scala

testcases/extern/GameSimpler.scala

testcases/extern/features.scala