Cache evaluators when filtering testbanks

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

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

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

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