Substitute Choose code with solutions

src/main/scala/leon/LeonContext.scala

src/main/scala/leon/Main.scala

src/main/scala/leon/plugin/CodeExtraction.scala

src/main/scala/leon/plugin/Extractors.scala

src/main/scala/leon/synthesis/SynthesisPhase.scala

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

testcases/synthesis/ChoosePos.scala