Enrich FreeableIterator's API, define GrowableIterable

src/main/scala/leon/datagen/SolverDataGen.scala

src/main/scala/leon/synthesis/ExamplesFinder.scala

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

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

src/main/scala/leon/utils/FreeableIterator.scala

src/main/scala/leon/utils/GrowableIterable.scala

...cala/leon/integration/solvers/ModelEnumerationSuite.scala