Remove Let/LetTuple madness, rely on constructors

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

src/main/scala/leon/synthesis/ConvertWithOracles.scala

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

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

...main/scala/leon/synthesis/rules/UnconstrainedOutput.scala