Synthesis framework refactoring

library/lang/string/String.scala

src/main/scala/leon/synthesis/BoundedSearch.scala

src/main/scala/leon/synthesis/CostModel.scala

src/main/scala/leon/synthesis/Distribution.scala

src/main/scala/leon/synthesis/ManualSearch.scala

src/main/scala/leon/synthesis/ParallelSearch.scala

src/main/scala/leon/synthesis/PartialSolution.scala

src/main/scala/leon/synthesis/Rules.scala

src/main/scala/leon/synthesis/SearchCostModel.scala

src/main/scala/leon/synthesis/SimpleSearch.scala

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

src/main/scala/leon/synthesis/SynthesisPhase.scala

src/main/scala/leon/synthesis/Synthesizer.scala

src/main/scala/leon/synthesis/Task.scala

src/main/scala/leon/synthesis/TaskRunRule.scala

src/main/scala/leon/synthesis/TaskTryRules.scala

src/main/scala/leon/synthesis/graph/DotGenerator.scala

src/main/scala/leon/synthesis/graph/Graph.scala

src/main/scala/leon/synthesis/graph/Search.scala

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