Multiple fixes to CEGIS and improvements of synthesis

src/main/scala/leon/solvers/z3/AbstractZ3Solver.scala

src/main/scala/leon/synthesis/ConversionPhase.scala

src/main/scala/leon/synthesis/ExamplesBank.scala

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

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

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

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

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

...scala/leon/integration/solvers/GlobalVariablesSuite.scala

testcases/synthesis/etienne-thesis/List/Delete.scala

testcases/synthesis/etienne-thesis/List/Diff.scala

testcases/synthesis/etienne-thesis/List/Insert.scala

testcases/synthesis/etienne-thesis/List/Split.scala

testcases/synthesis/etienne-thesis/List/Union.scala