Fix synthesis of type-parametric values

src/main/scala/leon/purescala/Constructors.scala

src/main/scala/leon/purescala/ExprOps.scala

src/main/scala/leon/purescala/Expressions.scala

src/main/scala/leon/purescala/TypeOps.scala

src/main/scala/leon/synthesis/Solution.scala

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

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

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

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

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