Introduce Witness, and Guide and Terminating as implementing classes

library/annotation/package.scala

library/lang/synthesis/package.scala

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

src/main/scala/leon/purescala/TreeOps.scala

src/main/scala/leon/repair/RepairCostModel.scala

src/main/scala/leon/repair/Repairman.scala

src/main/scala/leon/repair/rules/GuidedCloser.scala

src/main/scala/leon/repair/rules/GuidedDecomp.scala

src/main/scala/leon/synthesis/ChooseInfo.scala

src/main/scala/leon/synthesis/Problem.scala

src/main/scala/leon/synthesis/Witnesses.scala

src/main/scala/leon/synthesis/rules/Cegless.scala

src/main/scala/leon/synthesis/rules/Tegless.scala

src/main/scala/leon/synthesis/utils/Helpers.scala

src/main/scala/leon/utils/Library.scala