Introduce canBeSubtypeOf/ canBeSupertypeOf, fix issues

src/main/scala/leon/frontends/scalac/CodeExtraction.scala

src/main/scala/leon/grammars/FunctionCalls.scala

src/main/scala/leon/purescala/Constructors.scala

src/main/scala/leon/purescala/SelfPrettyPrinter.scala

src/main/scala/leon/purescala/TypeOps.scala

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

src/main/scala/leon/synthesis/utils/Helpers.scala

src/test/scala/leon/unit/purescala/TypeOpsSuite.scala