Unify and centralize use of solvers

src/main/scala/leon/codegen/runtime/ChooseEntryPoint.scala

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

src/main/scala/leon/solvers/SolverFactory.scala

src/main/scala/leon/synthesis/SynthesisContext.scala

src/main/scala/leon/synthesis/SynthesisSettings.scala

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

src/main/scala/leon/termination/Processor.scala

src/main/scala/leon/utils/Simplifiers.scala

src/test/scala/leon/test/synthesis/SynthesisSuite.scala