better printing of verification condition for loop invariant

src/main/scala/leon/Analysis.scala

src/main/scala/leon/DefaultTactic.scala

src/main/scala/leon/FunctionClosure.scala

src/main/scala/leon/ImperativeCodeElimination.scala

src/main/scala/leon/UnitElimination.scala

src/main/scala/leon/VerificationCondition.scala

src/main/scala/leon/purescala/Definitions.scala