Fix Solution unapply, some cleanup

src/main/scala/leon/solvers/string/StringSolver.scala

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

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

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

...n/scala/leon/synthesis/rules/unused/IntegerEquation.scala

...ala/leon/synthesis/rules/unused/IntegerInequalities.scala