Refactor simplifiers to take initial PC, make it a proper Path

src/main/scala/leon/purescala/ExprOps.scala

src/main/scala/leon/purescala/SimplifierWithPaths.scala

src/main/scala/leon/purescala/TransformerWithPC.scala

src/main/scala/leon/synthesis/Solution.scala

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

src/main/scala/leon/utils/Simplifiers.scala