Add ??? as function-scope holes, convert them to oracles

library/lang/synthesis/package.scala

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

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

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

src/main/scala/leon/purescala/Trees.scala

src/main/scala/leon/synthesis/ConvertHoles.scala

src/main/scala/leon/utils/PreprocessingPhase.scala

src/test/resources/regression/synthesis/Holes/Hole1.scala

src/test/resources/regression/synthesis/Holes/Hole2.scala