Refactor VerificationContext

src/main/scala/leon/laziness/LazyVerificationPhase.scala

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

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

src/main/scala/leon/verification/Tactic.scala

src/main/scala/leon/verification/VerificationCondition.scala

src/main/scala/leon/verification/VerificationContext.scala

src/main/scala/leon/verification/VerificationPhase.scala