Fix performance issue (generation of Pickler[Any])

* When generating (un)picklers, most of the time we look for the pickler
  of `Any`. By exposing the pickler of `Any`, we don't manually create
  one pickler/unpickler every time it's needed. This speeds up the
  compilation time almost by a factor of 2.

core/src/main/scala/scala/pickling/pickler/Any.scala

@@ -44,5 +44,5 @@ object AnyPicklerUnpickler extends AbstractPicklerUnpickler[Any]
 }
 
 trait AnyUnpicklers {
-  implicit val anyUnpickler: Unpickler[Any] = AnyPicklerUnpickler
+  implicit val anyPicklerUnpickler: AbstractPicklerUnpickler[Any] = AnyPicklerUnpickler
 }